Why does SPF's all= mechanism break when using a third-party email relay?

You send a campaign through SendGrid, and suddenly your emails aren’t landing in inboxes. You check your logs, and SPF alignment is failing. You’ve set up an all= mechanism in your SPF record, so why is it breaking?

SPF’s all= mechanism is meant to define the boundary of authorized sending servers. But when you use a third-party email relay like SendGrid or Amazon SES, the real sending server isn’t your own. It’s their infrastructure — outside your SPF record entirely.

Unless you explicitly include the relay’s IP addresses or use a mechanism like SPF alignment with a mechanism that trusts the relay’s authentication, the all= mechanism blocks the email. The result? Rejection, spam filtering, or delivery failure — even if your content is clean.

Key takeaways

  • SPF’s all= mechanism blocks any server not explicitly listed, including third-party relays, unless their IPs are included.
  • SendGrid, Amazon SES, and similar services operate outside your domain’s SPF record, breaking strict all= policies by default.
  • Using a relay requires explicitly authorizing its IPs in SPF or relying on a trusted authentication mechanism like Sender Policy Framework alignment with a compliant service.

How does the SPF all= mechanism work under normal conditions?

The SPF all= mechanism defines the default outcome when no other mechanism in the SPF record matches the sending server. For example, v=spf1 ip4:192.0.2.0/24 all means that only IPs in that range are authorized, and all others are blocked by default. This behavior assumes any unlisted IP is unauthorized — a dangerous assumption if you rely on third-party services like email relays.

SPF checks origin, not identity

SPF doesn't verify who sent the email — only that the server was authorized to do so. It’s a gatekeeper for the sending IP, not a trust signal for the email's authenticity. If an SPF record includes all without careful controls, it risks blocking legitimate mail if the origin IP isn’t explicitly listed.

That’s why SPF records often use all=reject or all=softfail to control policy. reject means the email is blocked. softfail means it may still be accepted but scored negatively by receiving systems. This gives senders flexibility — but also responsibility.

Why SPF all= can break with third-party relays

When you use a third-party email relay service (like SendGrid, Amazon SES, or Mailgun), their IPs are not your own. If your SPF record only lists your internal servers and not the relay's IPs, mail fails SPF verification — even if the email is legitimate. The all mechanism then blocks it outright.

This is why SPF alignment breaks: the email appears to come from your domain, but the sending server isn't in your SPF list. The all qualifier has no way to know the relay is trusted — it just sees an unlisted IP and rejects.

According to the IETF, SPF is designed to prevent spoofing at the IP level, not to endorse every valid sender. RFC 7208 states that SPF policies must be explicit. A misconfigured all can therefore cause valid email to be rejected — a key reason why large senders include multiple authorized services in their SPF records.

If you’re sending via third-party relays, you must list them — or risk high bounce rates and delivery failures. Tools like MailTester’s email checker can validate whether an address is valid before you send, reducing the risk of SPF-related failures on the sender side.

What happens when a third-party relay service sends on your domain's behalf?

When a third-party service like SendGrid, Mailchimp, or AWS SES sends email using your domain, the sending server’s IP won’t be listed in your SPF record, causing SPF checks to fail. Even if DKIM and DMARC pass, a strict SPF 'fail' or 'reject' mechanism can still block delivery, leading to undelivered messages or placement in spam folders. This is especially common when SPF includes all=reject or all=fail without explicitly allowing third-party relays.

SPF failures don’t disappear just because DKIM and DMARC pass

It’s a common mistake to assume that if DKIM and DMARC are properly set up, SPF doesn’t matter. But receiving servers evaluate each authentication mechanism independently. SPF failure alone can trigger rejection, even if both DKIM and DMARC pass. The receiver sees the sender’s IP as unauthorized for your domain, regardless of signature validity.

Take this example: your company uses Mailchimp to send a newsletter. Mailchimp’s servers send the email from your domain, but their IP address isn’t in your SPF record. If your SPF record includes all=reject, the receiving server will reject the email based on SPF alone—no matter how strong the DKIM signature or whether DMARC policy is aligned.

How to fix this — and why most people get it wrong

SPF doesn’t block all third-party sends by design — it’s meant to allow them when you explicitly configure them. The fix is to add the third-party’s sending IPs or their SPF include mechanism to your domain’s SPF record.

For instance, SendGrid’s current SPF includes are publicly documented and can be added via include:sendgrid.net. Similarly, Mailchimp uses include:mailchimp.com. If you’re using AWS SES, include include:amazonses.com. Leaving these out means your SPF is effectively broken for any outbound mail sent through those services.

Don’t assume SPF is "good enough" just because your domain sends some emails successfully. A single unverified relay service can trigger widespread delivery failures. According to RFC 7208, SPF failure is not optional — it’s a defined gatekeeper for email legitimacy. You can test whether your domain remains compliant across common relays with a real inbox placement test.

Check your domain’s SPF alignment and catch issues before they break campaigns. Use inbox placement testing to verify how your messages land across providers, or use email verification to validate each address before sending, reducing the risk of sending to invalid or misconfigured domains.

Can SPF all= ever be safely used with third-party relays?

No, SPF’s all= mechanism — especially with a fail policy like all - or all ~ — cannot be safely used with third-party email relays. It blocks legitimate mail from services like SendGrid, Mailchimp, or Amazon SES because those relays aren’t listed in your SPF record, leading to consistent authentication failures and delivery drops. The only safe way forward is to either remove the all mechanism entirely or explicitly include the relay’s authorized IPs via include or ip4/ip6.

Why SPF all= breaks with relays

When you set an SPF record with all -, you’re saying: “Only these listed sources are allowed. Everything else fails.” But third-party relays use different IP addresses than your domain’s origin servers. If you don’t include them, every email sent through the relay fails SPF validation, even if the message is legitimate.

Let’s say you send transactional emails via a service like SendGrid. Their IP ranges are not part of your internal infrastructure. Without an include:_spf.sendgrid.net directive, SPF will see the sending IP as unauthorized and reject the email. This isn’t a minor glitch — it’s a hard failure that triggers filtering and often results in permanent bounce-backs.

How to fix it: Structure SPF for relays

SPF is designed to be flexible. Rather than relying on all - for default rejection, you should use the include mechanism to explicitly authorize each third-party email sender. For example: v=spf1 include:_spf.google.com include:_spf.sendgrid.net ~all allows Google and SendGrid to send on your behalf while still signaling you have a policy (soft fail) for unknown senders.

Removing the all mechanism entirely isn’t recommended in most cases — it can weaken your email security posture. Instead, define your allowed sources clearly and let SPF work as intended: only fail when you’ve explicitly authorized a source. The SPF RFC (Section 4.6) confirms that all should only be used when you fully control all sending sources.

Before sending to a large list, verify your SPF alignment using a tool like MailTester’s inbox placement tester to ensure your emails don’t get flagged. This helps identify issues like missing relays or overly restrictive records before they damage sender reputation or trigger blocklists.

How to correctly configure SPF when using a third-party email relay

You must use v=spf1 as the version, then add include:_spf.sendgrid.net (or your relay’s domain) to reference their SPF record. Never use all= unless you fully trust every sender, which is risky. Keep the total number of mechanisms and includes under 10 to stay within DNS limits. Test your record with tools like MxToolbox or the SPF Validator at MxToolbox before sending.

Step-by-step SPF setup with third-party relays

  1. Start every SPF record with v=spf1. This tells receiving servers the version of the SPF protocol you're using. Without it, the record is invalid and ignored.
  2. Add the include mechanism for your email relay. For example, if you use SendGrid, include include:_spf.sendgrid.net. This delegates SPF validation to SendGrid’s proven policies.
  3. Avoid using all= or all unless you explicitly intend to allow all servers. Misconfiguring this can lead to deliverability failures or increase risk of spoofing.
  4. Keep the total number of mechanisms and includes under 10. Each include, ip4, ip6, or mx counts. Exceeding this limit invalidates the record.
  5. Use MxToolbox’s SPF Validator to test your record. It checks syntax, chain depth, and includes real-world behavior across providers.
  6. Check your public DNS record with RFC 7208, Section 8 to confirm it follows standard practices for alignment and delegation.

Common pitfalls to avoid

  • Don’t combine multiple include records for different services in one record unless necessary. Each adds to the mechanism count and risks hitting the limit.
  • Never rely on a relay's SPF record alone—your own domain must include it properly. Failure to do so breaks authentication.
  • Don’t add all= to "fix" issues. It weakens your security and invites abuse.

When in doubt, verify your SPF setup using a real-time tool. You can test your entire list for deliverability issues with MailTester’s inbox placement test, which simulates real-world delivery outcomes across major inboxes.

Why does SPF fail on relay-sent messages even with valid DKIM and DMARC?

SPF can fail on relay-sent messages even when DKIM and DMARC pass because DMARC checks alignment — not just presence. If the sending domain’s SPF record doesn’t include the relay’s IP, alignment fails, and DMARC policy enforcement can reject the message, regardless of valid signatures. This is why SPF configuration remains foundational, even with strong DKIM and DMARC.

Alignment is the gatekeeper, not just validation

DMARC doesn’t care if an email is signed or even if the DKIM signature is valid — it cares whether the domain in the From header aligns with the domain used in SPF or DKIM. If the envelope sender (MAIL FROM) is from your domain but the sending IP isn’t listed in your SPF record, alignment fails. A valid DKIM signature still won’t help here.

Spam filters and receivers evaluate SPF alignment independently. Even if DKIM passes and DMARC is configured, misalignment breaks the chain. This is why an email sent through a third-party relay like SendGrid or Mailchimp can be rejected — not because the message is fake, but because the sender’s IP isn’t trusted in your SPF record.

One failure, one rejection

DMARC policies are binary: pass or fail. Even if DKIM and DMARC are technically valid, a single failed alignment is enough for receivers to apply the DMARC policy — often quarantine or reject. This applies even if the email is sent via a well-known provider that uses proper authentication.

That’s why you must ensure your SPF record includes all third-party relays you use. Otherwise, the email path looks suspicious. For example, if your marketing emails go through Klaviyo, your SPF must include Klaviyo’s IPs. Otherwise, you’re trusting DKIM to cover for SPF, which it cannot.

Many receivers, including Gmail and Microsoft 365, use strict alignment checks. The RFC 7052 (which outlines DMARC) states that alignment is mandatory for policy enforcement — meaning no exceptions. You can’t skip SPF just because you have DKIM.

Use tools like inbox placement testing to simulate how your messages are treated across major platforms. If you’re sending through a relay and still seeing bounces or delivery failures, checking SPF alignment is the first step.

What roles do SPF, DKIM, and DMARC play in relay-based email delivery?

SPF, DKIM, and DMARC work together to verify that an email came from a legitimate source and wasn’t altered in transit. SPF checks the sending server’s IP against the domain’s published record. DKIM cryptographically signs parts of the message, proving it wasn’t tampered with. DMARC uses SPF and DKIM results to enforce policies—such as rejecting or quarantining messages—based on alignment between headers and the domain. When using a third-party email relay, misalignment in any of these components can trigger delivery failures, even if the message is technically valid.

How relay services impact SPF and DKIM alignment

When you send through a third-party relay like SendGrid, Mailgun, or Amazon SES, the sending IP changes. SPF relies on the IP being on the domain’s allowlist, so unless the relay’s IP is explicitly included in the SPF record, SPF will fail. This is why SPF records often include mechanisms like include:spf.provider.com — but if the record has all=reject and a missing include, it can block all outbound mail.

DKIM signs the message using a key stored on the sender’s domain. But many relay providers use a shared DKIM key they don’t publish in DNS. If the key isn’t published or if the selector doesn’t match, DKIM verification fails. Even when correctly signed, DKIM alignment fails if the signing domain doesn’t match the “From” domain in the message header, which is a common pitfall when relays aren’t configured for domain alignment.

Why DMARC can override SPF even with valid DKIM

DMARC evaluates SPF and DKIM results together. It applies the policy based on alignment, not just pass/fail. If SPF fails but DKIM passes, DMARC can still reject the message if the alignment isn’t correct—or, worse, the policy can be set to "none," which allows the message through despite a failed SPF.

Even if DKIM is valid, a weak SPF alignment can override it during DMARC evaluation. For example, if SPF fails due to a non-aligned IP but DKIM passes, DMARC applies the policy. If that policy is "reject," the message is blocked. But if the policy is "quarantine" or "none," the email may still land in spam or be rejected without a clear signal. This is why testing inbox placement and verifying email delivery paths is critical—especially with third-party relays.

Use tools like inbox placement testing to validate how your emails perform across real inboxes. Check SPF, DKIM, and DMARC settings using email address verification tools before sending. These checks help catch alignment issues before they impact deliverability. The goal isn’t perfection—it’s consistency at scale.

How to test if your SPF record works with your relay service

You can verify if your SPF record correctly authorizes your third-party email relay by sending a test email from the relay to a real inbox (like Gmail), then inspecting the full email headers for a 'pass' in the Received-SPF and Authentication-Results fields. If you see 'fail' or 'neutral', your SPF setup is not properly aligned with the relay’s sending domain.

Step-by-step verification process

  1. Send a test email via your relay service using a well-known delivery domain like you@ gmail.com. Choose a real inbox — not a test address — so the headers reflect actual delivery conditions. You're simulating real-world sending behavior.
  2. Download the full headers from the received message. On Gmail, click the three-dot menu in the email and select "Show original." The full header includes authentication results that show if SPF passed.
  3. Look for Received-SPF and Authentication-Results lines. The Received-SPF field indicates whether the sending IP or domain was authorized by your SPF record. The Authentication-Results field gives a summary of all authentication checks (SPF, DKIM, DMARC).
  4. Check the SPF result value. A correct setup will show pass or pass; mechanism=include. This confirms the relay’s sending IP is approved by your SPF record. If you see fail or neutral, the relay is not properly authorized.
  5. Confirm via DNS lookup using tools like dnschecker.org or RFC 7208 to validate that your SPF record includes the relay’s domain or IP range, and that there are no syntax errors or exceeding the 10-lookup limit.

Use third-party tools for faster validation

Let’s speed this up with a testing tool like Mail-Tester.com — it checks deliverability and shows the exact SPF result in a clean report. You can also send a test to MailTester's inbox-placement tool to simulate real inbox filtering and catch SPF issues before mass sends.

What are the risks of ignoring SPF configuration for third-party relays?

You risk message rejection by major providers like Gmail and Outlook—even with correct DKIM—if your domain’s SPF record doesn’t explicitly allow the third-party email relay service you’re using. SPF failures can trigger high bounce rates, degrade sender reputation, and put your domain on blocklists like Spamhaus, especially if failures are repeated. Even if DKIM signs your messages properly, SPF remains a hard gate; a failure here blocks delivery entirely.

Common consequences of SPF misconfiguration

  • Messages are rejected by receivers that enforce SPF strictly, which includes most modern email providers — especially Gmail and Outlook. A failed SPF check typically results in a hard bounce.
  • Repeated SPF failures signal poor email hygiene to reputation systems. Over time, this can reduce inbox placement even for valid messages, leading to decreased campaign performance.
  • High bounce rates and spam complaints (even if unintentional) can trigger automated filters, increasing the risk of being flagged on public blocklists like Spamhaus or SURBL.
  • Third-party services like Mailchimp, SendGrid, or HubSpot must be explicitly included in your SPF record using include: mechanisms. Omitting them breaks SPF verification entirely, regardless of other valid configurations.
  • Some relay services use multiple IP ranges or domains. If your SPF record only lists one, it won’t cover all outbound routes, causing intermittent delivery failures.
  • Even if DKIM is correctly set, receivers often apply SPF as an initial filter. A failed SPF check can override a passing DKIM, blocking the message before it’s even analyzed for content.

How to avoid these risks

Let’s be clear: SPF isn’t optional. It’s a foundational email validation mechanism. You should verify your current SPF setup regularly using tools that test both record syntax and real-world delivery behavior.

  • Use a real-time email verification service to test how your messages are received. For example, MailTester’s inbox placement test helps identify if SPF failures are blocking delivery in practice.
  • Check your SPF record against standards in RFC 7208 — the protocol defining SPF — to ensure all includes, mechanisms, and limits are valid and within the 10-lookup limit.
  • Always include third-party relays explicitly with include: — never rely on their default configuration. If you use SendGrid, add include:sendgrid.net.
  • Test your records using tools like MXToolbox to validate syntax and check for common errors like over-lookup limits or missing mechanisms.
  • Never assume that DKIM alone is enough to bypass SPF. It isn't. Both are required for high deliverability.

How can MailTester help prevent SPF-induced delivery failures?

MailTester catches SPF-related delivery risks before they hit your inbox. By verifying email addresses at scale, testing inbox placement across major providers, and identifying malformed SPF records or risky addresses, MailTester ensures your sends aren’t blocked by alignment failures or third-party relay issues. You avoid wasting sends on invalid or catch-all addresses that trigger SPF rejections.

Bulk verification stops invalid addresses before they cause issues

You don’t need to guess which addresses will fail. MailTester’s bulk verification checks thousands of emails in minutes, flagging invalid, malformed, or unverifiable addresses—many of which would otherwise trigger SPF rejections when relayed through third-party services. This upfront cleanup prevents deliverability issues before they happen.

Test real-world inbox placement to catch SPF blocks early

SPF failures don’t always show up as hard bounces—sometimes, messages are silently dropped or marked as spam. MailTester’s inbox placement tests simulate real delivery across Gmail, Outlook, Yahoo, and other major providers. These tests detect SPF alignment mismatches that might not trigger an error in the SMTP handshake but still block delivery. You see exactly where your messages land—and why.

When you send through a third-party relay like SendGrid or AWS SES, SPF alignment depends on whether the sender domain matches the authorized sending domain. If your list includes addresses from domains with overly permissive or misconfigured SPF records (e.g., all= allowing any domain), MailTester detects those as risky. The SPF record could be bypassed by a relay service, leading to rejection. MailTester flags these before you send.

Using the MailTester API during list hygiene lets you catch malformed SPF records in real time. For example, an SPF record with syntax errors like missing quotes or conflicting mechanisms won’t pass alignment checks. You can integrate verification into your workflow—whether it’s before a campaign or within a CRM sync—to catch issues before they compound.

Let’s be honest: SPF alignment is complex. You can’t rely solely on email clients to report what’s wrong. MailTester’s in-app AI assistant analyzes raw email headers, identifies alignment gaps between From and Return-Path, and explains why a message might be blocked—often pointing to misconfigured third-party relay setups. This real-time insight saves hours of manual debugging.

Learn more about how SPF works at the IETF’s SPF specification—the foundation of email authentication. Still, no standard alone prevents all delivery failures. That’s why proactive verification with tools like MailTester is essential.

SPF all= mechanism override is not a feature — it’s a configuration misalignment

The notion that a third-party email relay service "overrides" SPF is incorrect. SPF is enforced by the receiving mail server based on the sender’s DNS records. Relay services do not alter this behavior.

An SPF all= mechanism does not automatically include third-party services. Without explicit inclusion (e.g., using +include:example.com), the relay is not trusted by receivers, leading to authentication failures.

Proper SPF configuration is non-negotiable for inbox placement. Even with a reliable relay, misconfigured SPF remains a common reason for email rejection or spam filtering.

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Frequently asked questions

Does a third-party email relay override SPF all=?

No. The relay doesn’t override SPF — it simply sends from an IP not listed in your SPF record. If the record includes 'all', it still enforces rules. The mechanism isn’t bypassed; the record is wrong.

Can I use SPF all= with SendGrid or Amazon SES?

Not safely. Using 'all' without including the relay’s SPF record causes consistent failures. You must use 'include' directives to authorize the relay’s IPs.

What happens when SPF fails with a third-party relay?

Messages may be rejected, marked as spam, or delayed. Even if DKIM passes, SPF alignment failure can trigger DMARC rejection.

How do I fix SPF errors when using a relay service?

Use 'include' to reference the relay's SPF record (e.g., 'include:_spf.sendgrid.net'). Avoid 'all' altogether unless you explicitly allow all servers.

Can I have multiple SPF records for one domain?

No. A domain can have only one SPF DNS record. Multiple records cause validation failure. Combine all mechanisms into a single record using 'include' directives.

Do I need to modify SPF if I change email providers?

Yes. Each provider has its own authorized IPs. You must update your SPF record to include the new provider’s 'include' directive or IP ranges.

Why does my email pass DKIM but still fail deliverability?

Because SPF alignment can fail even if DKIM passes. DMARC requires alignment from both mechanisms. A single failure can trigger rejection.

Is SPF still relevant with modern email deliverability systems?

Yes. SPF is still a foundational layer in sender reputation. Many providers use it as a first-line check. Ignoring it leads to delivery failure.

How do I test my SPF record for a relay service?

Use header analysis tools like mail-tester.com or MxToolbox. Send a test email and inspect the 'Received-SPF' and 'Authentication-Results' headers for pass/fail.

Can MailTester detect SPF misconfiguration issues?

Yes. MailTester’s inbox placement tests include SPF alignment checks across providers. Its bulk verification can help identify addresses that expose configuration risks.