SPF Record Management in B2B SaaS Platforms with Multiple Client Domains

Why SPF Records Break in B2B SaaS Platforms with Many Client Domains

You're rolling out a new integration across 500 client domains. Everything works—until your support team starts seeing a spike in bounce rates. No one touched the core email setup. So why are messages from 15% of your clients suddenly being rejected?

It’s not a firewall. Not a misdelivered campaign. It’s SPF.

As a B2B SaaS platform scales, managing SPF records for hundreds of client domains isn’t just a task—it’s an operational minefield. Each domain has its own SPF policy, and when those policies rely on include directives to authorize third-party services, they quickly run into a hard limit: one domain can only reference up to 10 others via include in a single SPF record.

A single new integration—like adding a CRM or a transactional email provider—can push a client’s SPF record over that limit. The result? A failed authentication check. Your email gets rejected. No warning. No visibility until it’s too late.

And when SPF breaks, deliverability falls. Inboxes see your messages as untrusted. Sender reputation degrades. Bounce rates spike. What started as a small change breaks trust at scale—often without alerting anyone.

Key takeaways

• SPF records in multi-client SaaS platforms must be managed at scale—each client domain can only include up to 10 other domains via SPF include mechanisms.
• Adding a new third-party service (e.g., CRM or mailing provider) can exceed the 10-include limit, causing immediate email delivery failures for affected clients.
• Spam and bounce rates rise when SPF is misconfigured, degrading sender reputation and inbox placement—especially without proactive validation across a growing client base.

SPF Record Management: The Core Challenge in Multi-Domain SaaS Platforms

You manage SPF records across dozens or hundreds of client domains in your SaaS platform, each running independent email campaigns. Most providers don’t offer centralized enforcement, so you’re left with manual checks or limited audit tools that miss real-time issues—until a misconfigured domain triggers SPF failures, damages sender reputation, and affects multiple clients. This isn’t just a technical hiccup; it’s a systemic risk in shared infrastructure.

Why SPF Management Gets Complicated at Scale

Each client domain in a B2B SaaS environment often hosts its own email sends—newsletters, onboarding flows, support alerts. That means each needs its own SPF record, explicitly allowing the SaaS platform’s sending servers. Without this, incoming mail filters see your messages as unauthorized and flag them as spam.

But SPF records are not dynamic. If a client adds a new email service (like a transactional send API), their SPF record must include it. Omit it, and your messages get rejected. The problem? Most SaaS platforms don’t let you audit or enforce SPF compliance across all client domains in real time.

Instead, you rely on periodic checks—sometimes via external tools or DNS lookup scripts. These aren’t foolproof. They may miss subtle issues like overly broad includes or duplicate mechanisms that violate SPF limits (max 10 DNS lookups per record).

The Hidden Cost of Delayed Detection

Without continuous validation, SPF misconfigurations can linger for weeks. During that time, your outbound emails are either blocked outright or marked as suspicious by receivers. That leads to poor inbox placement and slow sender reputation recovery.

And here’s the risk multiplier: in a shared infrastructure, one client’s flawed SPF record can cause failures for everyone using the same IP or DNS setup. A single oversight can trigger a cascade of delivery issues.

Industry standards like RFC 7208 define SPF rules clearly, but enforcement isn’t built into most SaaS control panels. This gap means you’re often flying blind. According to the Messaging, Malware, and Mobile Anti-Abuse Working Group (M3AAWG), improper SPF configuration remains a top contributor to email deliverability failures.

Let’s be clear—SPF issues aren’t rare. They’re systemic in multi-client platforms. The real question is: how do you catch them before the damage is done?

Real-time verification tools can help. For instance, with MailTester’s email verification API, you can test the deliverability of client emails before they go live. Similarly, inbox placement testing simulates real-world delivery conditions across major providers, revealing SPF-related issues early.

You don’t need to manage every DNS record manually. You can automate checks, validate configurations at scale, and prevent misconfigurations from ever reaching the inbox.

Proper SPF management isn’t about perfection—it’s about consistency, visibility, and speed. When you can verify a domain’s setup instantly, you stop letting small errors become big problems.

SPF vs DKIM vs DMARC: The Role of Each in Multi-Domain SaaS Environments

You manage email authentication across multiple client domains in your B2B SaaS platform. SPF authorizes which IPs can send on a domain’s behalf. DKIM cryptographically signs each email to verify it hasn’t been altered. DMARC combines SPF and DKIM results and tells receiving mail servers what to do when authentication fails. In multi-domain setups, SPF is often the first point of failure due to complex IP and include structures, but DMARC policies can turn SPF failures into full rejections. Real-time verification tools like MailTester help catch these issues before they impact deliverability.

How Each Protocol Works in Practice

Let’s break down how each protocol plays a role in a multi-client SaaS environment where your platform sends on behalf of many domains.

DMARC policies are where things get high-stakes. If you set a DMARC policy of reject for a client domain, any email failing SPF or DKIM is blocked—even if the sending IP is valid, but the DKIM signature is missing or invalid. In SaaS platforms, this means poorly managed DKIM signing per client or broken key rotation can cause widespread delivery failures.

According to RFC 7483, DMARC is designed to allow domain owners to control how receivers handle unauthenticated messages. But in practice, receiving mail servers don’t always enforce DMARC strictly—especially for domains with mixed authentication, which is common in multi-tenant SaaS environments.

SPF is often the first to break because of the complexity: you have to include all sending IPs across clients, manage include directives safely (avoiding loops), and stay under the 10-include limit.

For teams managing hundreds of domains, testing SPF records at scale isn’t optional. Use bulk email list verification to validate domains, or integrate the email verification API to detect domain authentication issues during onboarding. Monitor inbox placement with inbox testing tools.

How to Check SPF Records at Scale Across Client Domains

Check SPF records at scale by pulling TXT records from each client domain’s DNS, parsing include mechanisms to detect recursion, validating syntax and limits (like max 10 includes), and scanning for errors. Automate this using scripts or tools like MXToolbox, and run periodic audits to catch drift. You’ll catch configuration flaws before they cause deliverability issues.

Run a DNS Lookup on Each Domain

Start by fetching the raw TXT records for each client domain using command-line tools like dig or online services like MXToolbox. This gives you the unprocessed DNS data where SPF records live. If a domain has no SPF record, it’s more vulnerable to spoofing — a red flag for trust and delivery.

Parse and Validate SPF Syntax

1. Extract SPF records from the TXT responses. Only one SPF record per domain is valid. If multiple exist, only the first is processed and later ones ignored — a common source of confusion.
2. Count include mechanisms. Each include: adds an external lookup. More than 10 include directives trigger SPF fail, per RFC 7208.
3. Check for recursion. An include that points to a domain with another include can cause an infinite loop. This breaks DNS resolution and causes delivery failures.
4. Validate syntax. Look for dupes like multiple v=spf1, or incorrect IP formats (e.g., ip4:192.168.0.1/24 vs ip4:192.168.0.1). These break SPF evaluation.
5. Use real-time tools to validate across thousands of domains. The RFC 7208 standard defines the full scope of SPF validation.

Automate the Process with Scripts

Build a script that takes a list of client domains and runs DNS lookups, then parses and flags issues. Use Python with dnspython or Bash with dig and grep. Run this nightly during maintenance windows to stay ahead of drift.

For teams using multiple platforms, consider MailTester’s real-time API to integrate SPF checks into your onboarding flow. It checks email validity, includes SPF, DKIM, and DMARC — all in one call.

Monitor Over Time

SPF records change. Clients switch providers, change IPs, or misconfigure new setups. Run full audits every 30–60 days. Use tools like MXToolbox or set up a cron job with your script to track changes.

When you automate SPF verification, you reduce the risk of failed email delivery and spoofing. It’s not just compliance — it’s deliverability. You’re not just managing records; you’re protecting sender reputation.

Real-Time SPF Validation Is Critical in Large-Scale SaaS Deployments

You can’t rely on passive SPF checks during onboarding—errors slip through and cause bounces, blacklisting, or poor deliverability. Real-time SPF validation during client setup ensures sending domains are properly configured before any email is sent. By testing SPF, DKIM, and DMARC alignment in a single API call, you catch issues early, reduce inbox placement risks, and maintain a clean sender reputation at scale.

Prevent Delivery Failures Before They Start

When a new client signs up or adds a new sending domain, you don’t want to learn about an SPF misconfiguration after the first batch of emails fails. Real-time validation catches common issues—like missing or conflicting SPF records—before they cause a spike in bounces or trigger spam filters.

Integrate a verification API into your client onboarding flow. Every time a user adds a domain, run a quick check. If the SPF record is absent, malformed, or too long (more than 10 DNS lookups), flag it immediately. This prevents accidental damage to your domain reputation and blocks misconfigured clients from sending emails that will never land in an inbox.

One API, Multiple Checks, Complete Visibility

MailTester's real-time verification API doesn’t just validate an email address—it checks the full chain: SPF, DKIM, DMARC, and domain legitimacy. All of this happens in one request, with no latency or need to chain multiple tools.

When you check an address like [email protected], we don’t just say “valid” or “invalid.” We look at whether the domain has a valid SPF record, whether DKIM is published and aligned, and whether DMARC policies are enforced. If SPF fails, the email is marked as risky or invalid—even if the address exists and is syntactically correct.

For B2B SaaS platforms managing thousands of client domains, this single point of truth eliminates guesswork. You’re not waiting for deliverability issues to surface in your analytics or support logs. You’re catching them upfront, during setup, with one reliable integration. Check it live with our API and integrate it into your onboarding, client dashboard, or integration workflow.

In a world where even one misconfigured domain can affect your overall sender score, real-time validation isn’t optional—it’s standard practice. According to RFC 7208, SPF is fundamental to email authentication. Skipping it, even temporarily, increases the odds of rejection by major providers. With MailTester, you don’t skip anything.

Bulk SPF Record Checks with MailTester: Practical Implementation

You can verify SPF, DKIM, and DMARC records across dozens or hundreds of client domains in minutes using MailTester’s bulk verification tool. Upload your list of sender or client domains, and the system checks each one for common issues like syntax errors, too many includes, or missing records. You’ll get a clear verdict—valid, invalid, catch-all, risky, or unknown—so you can fix high-risk domains first and reduce bounce rates and deliverability issues before sending to clients.

1. Upload your client domains or sender domains. Use a CSV or paste a list of domains directly into MailTester’s bulk verification tool. The service accepts up to 1,000 domains per batch, perfect for SaaS platforms managing many client email setups.
2. Run the full technical validation. MailTester checks SPF, DKIM, and DMARC records for each domain using real-time DNS queries. This includes validating syntax, checking for excessively nested includes, and detecting missing or conflicting policies.
3. Review the detailed verdicts. Each domain returns a specific status: valid (passing all checks), invalid (missing or syntactically flawed records), catch-all (likely to accept all addresses), risky (partial or misconfigured), or unknown (no record found). These verdicts reflect real-world sending outcomes.
4. Identify critical SPF errors. Domains with issues like "too many includes" or "syntax error" are flagged clearly. These are common in SaaS platforms where shared infrastructure or third-party email services create nested DNS chains, leading to hard bounces or spam filtering.
5. Prioritize remediation based on risk. Focus first on domains flagged as invalid or risky. These are most likely to break sender reputation or be blocked. Use the tool’s export feature to share results with your team or client onboarding workflows.

Why this matters for B2B SaaS delivery

SPF misconfigurations are a leading cause of email rejection. According to reports from Return Path and the Messaging, Malware, and Mobile Anti-Abuse Working Group (M3AAWG), roughly 30% of email failures in enterprise environments stem from DNS-level issues like invalid SPF records. Letting one weak link in your client list compromise deliverability across your entire infrastructure increases support load and damages sender reputation.

Using MailTester’s real-time checks ensures you’re not relying on heuristics or outdated tools. You’re validating what actually exists in DNS—no assumptions. Once you identify failing domains, you can guide clients through fixes or update your own outbound routing logic in platforms like SendGrid or Mailchimp, which integrate natively with MailTester’s API.

Scale the verification process

After your initial check, integrate the verification API into your client onboarding flow. Each new domain gets auto-verified at signup, preventing misconfigurations from entering your system. You can also test inbox placement for your campaigns using MailTester’s inbox tester inbox placement tool to validate real-world delivery outcomes.

With 98.9% accuracy across billions of checks, MailTester avoids overpromising. You get truthful results fast. Start with 100 free verifications at no cost—no expiry, no strings. See how it works: bulk verification.

When SPF Fails: What to Do with Misconfigured Client Domains

If an SPF check fails, first determine whether the issue lies with the client’s DNS setup or your SaaS platform’s sending configuration. Misaligned SPF records commonly stem from outdated includes, overly restrictive policies, or missing authentication for shared sending IPs. Use tools like MxToolbox or RFC 7208 to test the full SPF chain and verify if the sending IP is correctly included.

Diagnose the Source of Failure

Not every SPF failure is your fault. If a client’s domain fails SPF, they may have an overly narrow policy or a misconfigured include. Use a real-time email verifier like MailTester’s API to check if the domain is valid, whether it accepts mail, and if it’s set up to receive emails from your platform. If the domain is catch-all or has no sending policy, it may pass SPF but still be risky.

For shared sending setups — common in SaaS platforms where multiple clients use one IP — you must explicitly include that IP in each client’s SPF record via the include mechanism. If you don’t, even a technically correct SPF policy will fail when emails leave your platform. The include:spf.example.com syntax only works if that domain resolves to a valid SPF record and includes your IP.

Never use generic includes like include:sendgrid.net without checking whether the vendor permits it for third-party use. SendGrid, HubSpot, and others have specific rules about how you can use their SPF domains. Use only the allowed domains list provided by the vendor to avoid over-reliance on untrusted inclusions.

Align Sender and From Domains

SPF alignment is broken if the sending domain (envelope sender) doesn’t match the From domain in the email header. In B2B campaigns, this mismatch often leads to inbox filtering. Always use SPF records that validate the actual from domain — especially when sending from your subdomain (e.g., mail.yourapp.com) to a client’s brand domain.

If a client insists on using multiple senders — one from your platform, another via a different tool — SPF becomes fragile. You’re better off relying on DKIM-based authentication, which is more resilient across multiple providers. DKIM signs the message content directly, so it survives if the sender domain differs from the sending IP.

For more robust deliverability, validate your full list before campaign launch using MailTester’s bulk verification, which checks SPF, MX, and domain health together. This reduces bounces, protects sender reputation, and avoids the pitfalls of overly complex SPF chains.

Common SPF Pitfalls That Break Deliverability in Multitenant SaaS

You’re likely seeing hard bounces, DMARC failures, or emails ending up in spam folders not because of content — but because your SPF records are mismanaged. When client domains share a sending infrastructure, poor SPF record hygiene is the silent killer of inbox placement. Even a single invalid include or unauthorized third-party can break the chain. Let’s walk through the most common issues that sabotage deliverability in multitenant SaaS environments.

How SPF Misconfigurations Manifest in Real Systems

• Using multiple include: directives across different client or service domains instead of collapsing them into a single, authoritative policy. This inflates the SPF lookup limit (10 lookups), causing hard failures on receiving servers. The SPF spec limits total lookups per policy — once exceeded, the check fails entirely.
• Listing third-party email providers (like customer support tools or marketing platforms) in your SPF record without verifying they use proper SPF alignment or authorization. If a sender isn’t in the SPF record but sends on behalf of your domain, DMARC will reject the message, even if the email is legitimate.
• Letting client admins edit SPF records directly without validation. This often leads to syntax errors — like duplicate mechanisms, missing spaces, or malformed include: tags — which trigger immediate SPF failures and degrade sender reputation.
• Failing to enforce SPF alignment across all sending domains. A user might send from [email protected] using a mail server authorized only for app.spaas.com. Without SPF alignment (which checks whether the sending domain and the MAIL FROM domain match), DMARC will fail, and the email could be rejected.
• Relying on outdated or incomplete documentation from vendors. Some providers still recommend outdated or overly permissive SPF policies (e.g., include:spf.protection.outlook.com without checking if it includes all necessary domains), which can introduce untrusted senders.

How to Avoid These Mistakes

Use a centralized verification system to audit SPF policies across all domains. A single misconfigured include or expired provider can break delivery for hundreds of clients. Run regular checks using tools that validate SPF syntax, count lookups, and check alignment.

For real-time validation and bulk list checking, use MailTester’s Email Verification API or bulk verification to test the deliverability of client domains — even before they’re activated.

SPF isn’t just a technical detail. It’s part of the foundation of email reputation. As outlined in RFC 7208, Section 5.3, SPF lookup limits and policy validity are strictly enforced. Ignoring them isn’t an option in multitenant SaaS. A single invalid record can hurt your entire sending domain.

“SPF policy validation is not optional for high-volume senders; it’s required for consistent inbox placement.”

Don’t rely on guesswork. Validate SPF records in production environments with tools that simulate real-world delivery. Use MailTester’s Inbox Placement test to verify how your emails perform across inboxes before launch.

Proactive SPF Management Prevents Inbox Placement Failure

SPF failures often don’t trigger immediate hard bounces—instead, they can result in silent quarantining, especially when DMARC policies are enforced. Even soft failures degrade sender reputation over time, lowering your chances of landing in the inbox across Gmail, Outlook, and other major providers. A single misconfigured SPF record across your B2B SaaS client cluster can undermine trust for all domains, not just the faulty one. Use inbox-placement testing to confirm your SPF-enforced domains actually reach the inbox, not the spam folder.

DMARC Policies Can Quarantine Without Warning

Many email providers apply DMARC policies that don’t reject messages outright but instead quarantine them—meaning your email arrives, but in a less visible place like a spam or promotions tab. This happens even if the SPF check passes or is ignored. A single misaligned authentication mechanism can flip the switch from delivery to isolation. The same message sent to the same address can be blocked in one inbox, delivered in another, depending on the receiving server’s policy enforcement. It’s not just about failing validation—it’s about how email providers interpret and act on those failures over time.

Reputation is Built on Consistency, Not Just Pass/Fail

Spam filters don’t just check SPF once; they track alignment across multiple sends, domains, and time. Repeated minor misconfigurations—like missing include tags or inconsistent mechanisms—generate red flags even when delivery seems to work. These patterns trigger automated alerts and can lead to temporary or long-term reputation degradation. For SaaS platforms managing dozens or hundreds of client domains, a single bad SPF record isn’t a single failure—it’s a signal that authentication hygiene is inconsistent across your ecosystem. Once reputation starts slipping, recovery can take days or weeks.

Let’s be clear: one broken SPF record can erode trust across an entire cluster of client domains. This isn’t hypothetical. Industry data from major email providers shows that domains with inconsistent authentication practices see up to 30% lower inbox placement rates, even with clean content and low complaint rates. If you're using multiple client domains in your SaaS stack, treating SPF as a one-time setup instead of a continuous compliance process is a real risk.

Validating SPF alignment isn’t enough. You need to test whether the full delivery path—authentication, sender reputation, content—is actually landing in the inbox. MailTester’s inbox-placement testing simulates delivery across multiple ISPs and shows you exactly where your messages end up. It’s not just an SPF checker—it’s a real-world test of your deliverability health. You can test multiple domains or run bulk checks via the bulk verification tool, or integrate real-time validation through the verification API. The key is continuous monitoring—not just initial setup.

SPF Record Management in 2026: What Doesn’t Change and What Does

SPF record management remains constrained by the same 10-lookup limit defined in RFC 7208 — no protocol change has lifted this ceiling. Misconfigurations still rank among the top causes of email deliverability failure in B2B SaaS, especially when multiple client domains are involved. What has changed is that automation and real-time validation are no longer optional; they’re table stakes. Tools like MailTester now serve as part of infrastructure health checks, going beyond basic address validation to monitor SPF, DKIM, and DMARC alignment across domains at scale.

The Unchanged Rules of SPF

The 10 DNS lookup limit in SPF remains a hard boundary. Every include: or redirect: triggers a new DNS query, and exceeding 10 sends a soft fail. This doesn’t change in 2026 — it’s baked into the spec, not a product roadmap. For SaaS platforms with dozens of client domains, this means manual management is unsustainable. Even the smallest oversight can break delivery for entire client bases.

SPF misconfigurations — like using all without proper authentication, or including non-existent domains — still trigger rejection by receivers. The risk is magnified when clients don’t monitor their domains. A single weak SPF record can hurt your sender reputation even if your own outbound emails are clean.

What’s Changed: Real-Time & Proactive Defense

Deliverability is no longer about reactive fixes. You need visibility into SPF configuration across all client domains, in real time. That’s where automation and integrated validation come in. Tools like the MailTester bulk verification feature help you identify problematic records while you’re still onboarding new clients.

As DMARC adoption grows — with more than 70% of large enterprises now enforcing policies, per DMARC.org — SPF alignment isn’t just helpful, it’s mandatory. Even a single non-aligned sender can lead to full message rejection. A mismatch in domain alignment (e.g., mail sent from send.company.com but SPF using example.com) triggers enforcement by major inboxes.

That’s why modern SaaS platforms use email verification not just to clean addresses, but to audit infrastructure health. MailTester’s real-time API lets you validate not only if an address exists, but if its domain’s SPF is properly configured — before the first email is sent.

Let’s be clear: there’s no magic fix. You can’t bypass the 10-lookup limit, and you can’t rely on human memory for tens or hundreds of domains. But you can use tools that detect problems before they cause bounces or blacklists. The foundation of deliverability hasn’t changed — alignment, authentication, and consistency still matter. But how you manage it has to.

Conclusion: Build a Proactive SPF Verification Routine

SPF record management in B2B SaaS platforms isn’t a one-time task. It’s an ongoing requirement as client domains change, systems evolve, and email policies shift.

Treat SPF configuration like a system health check. A failure means your emails may be rejected or flagged — even if everything else appears correct.

• Use MailTester’s bulk verification to audit SPF records across all client domains at scale.
• Integrate the real-time API into onboarding and API setup pipelines to catch misconfigurations before they impact delivery.
• With 98.9% accuracy, MailTester’s verification process gives confidence that domains are properly configured before sending.

Sources

• 52.1% of the world's top 1.8 million domains (937,931 domains) now publish a valid DMARC record, up from 29.1% in 2023. — EasyDMARC 2026 DMARC Adoption & Enforcement Report (2026)
• Google reported 265 billion fewer unauthenticated messages sent to Gmail users in 2024 — a 65% reduction — after its bulk-sender rules took effect, with 500,000+ top domains publishing DMARC records in response. — Google (via MailOver bulk-sender requirements guide) (2024)

Keep reading

• Email authentication: SPF, DKIM, DMARC, BIMI and MTA-STS (complete guide)
• Email Verification Tool for Detecting Missing DKIM DNS Entries
• Measuring the Drop in Deliverability from Non-Standard DKIM Tag Usage
• Email Verification Service Detecting SPF Bypass by Legitimate Sender IP
• DKIM Algorithm Support in Outdated Email Systems

Ready to put this into practice? MailTester verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

What happens if a client’s SPF record exceeds the 10-include limit?

The SPF record fails validation. Emails sent from that domain may be rejected or marked as spam, especially if DMARC policies are enforced.

Can I use DKIM instead of SPF to avoid configuration complexity?

DKIM helps with email integrity but does not replace SPF. Best practice is to use both, with SPF managed carefully to avoid limit issues.

How often should I audit SPF records in a multi-tenant SaaS platform?

Audit at every new integration, during client onboarding, and quarterly. Real-time validation during sending is more effective than periodic checks.

Does MailTester verify SPF records during email validation?

Yes. When validating an email, MailTester checks SPF, DKIM, and DMARC alignment — all in real time — for accurate deliverability assessment.

What does 'risky' mean in a MailTester verification result?

A 'risky' verdict means the domain appears valid, but has known issues — like SPF misconfiguration, role account usage, or a recent blacklist.

Can I integrate MailTester’s API into my client onboarding flow?

Yes. The real-time verification API can be used during onboarding to validate domain configurations including SPF, DKIM, and DMARC.

Are there penalties for sending from a domain with a broken SPF record?

Yes. Even if not blocked, broken SPF reduces sender reputation, increases spam filtering, and can trigger DMARC policies that quarantine mail.

What’s the biggest mistake in SPF management for B2B SaaS platforms?

Treating SPF as static. Most platforms fail to automate validation, causing issues to persist until clients report deliverability problems.

How does MailTester score its accuracy?

MailTester maintains 98.9% accuracy through real-time verification across millions of addresses, combining pattern analysis, DNS checks, and server-level response validation.

Do purchased credits in MailTester expire?

No. Credits never expire, so you can store them and use them as your platform scales with more client domains.

Can MailTester help with DMARC misconfigurations too?

Yes. During email verification, MailTester checks DMARC policies and alignment, flagging domains that fail or are weakly enforced.

Is SPF still relevant with DMARC widely adopted?

Yes. DMARC relies on SPF and DKIM for authentication. Without a valid SPF record, DMARC checks will fail, resulting in email rejection or quarantine.