Why does SPF include recursion depth matter in Microsoft 365 email verification?

You send a campaign through Microsoft 365, and half your emails vanish into the void. No bounce, no error — just silence. You check your SPF record, it passes every test. So why does M365 reject the message?

The issue often lies in recursion depth: Microsoft 365 enforces a hard limit of 10 nested include mechanisms in SPF records. Exceed that — and validation fails, even if your record appears valid in a generic verifier.

SPF isn’t just a syntax checker. It resolves DNS chains, following each include to fetch external policies. Too many layers, and the process can loop infinitely or time out. That’s why M365 caps recursion at 10 — not to complicate things, but to stop abuse and protect the system.

Key takeaways

  • Microsoft 365 enforces a strict recursion depth limit of 10 include mechanisms in SPF records.
  • SPF validation can pass in tools that ignore this limit, leading to false positives and undetected send failures.
  • Exceeding the recursion depth results in email rejection, even if the SPF record appears syntactically correct.

How does the SPF include mechanism recursion depth limit affect deliverability?

If your SPF record exceeds 10 include mechanisms in Microsoft 365, it triggers a permanent SPF validation failure (permerror), which harms deliverability even if DKIM and DMARC pass. M365 doesn’t bounce the email immediately but treats it as suspect—routinely filtering messages into spam or rejecting them outright. This happens regardless of other authentication success because M365 enforces SPF strictly, especially for complex, deeply nested records.

Why recursion depth matters in SPF

SPF uses a series of include directives to reference other domains' SPF policies. Each level of inclusion counts toward a hard limit: Microsoft 365 stops validating after 10 nested includes. If you exceed that, the validation fails at the 11th level—this is a permerror, not a temporary issue.

Let’s say you have a chain like: include:provider1.com → include:vendor1.com → include:subcontractor.com, and so on. By the 11th layer, even if all policies are valid, M365 stops evaluating. The result? An SPF fail, and that’s enough to trigger inbox filtering.

Deliverability impact despite valid DKIM and DMARC

DKIM and DMARC are independent of SPF. You can pass both and still fail SPF due to recursion depth. M365 treats SPF validation as a gatekeeper. When it fails, even with signed and aligned messages, the sender reputation takes a hit. This can lead to reduced inbox placement, especially for new or less established senders.

According to industry practices documented in RFC 7208, the SPF specification defines a maximum of 10 include iterations. Microsoft 365 implements this limit strictly. This isn’t a flaw—it’s a deliberate design to prevent abuse and protect against DNS lookup floods. RFC 7208 specifies these limits, and many ESPs enforce them.

If you're managing email campaigns or sending transactional messages through M365, verifying your SPF structure is essential. Deeply nested includes are common in enterprise environments, especially when third-party tools or legacy systems are involved. Even a single over-nested policy can jeopardize deliverability.

To avoid this, regularly audit your SPF records. Use tools like MailTester’s bulk verification to test domains for SPF issues before sending. You can also check individual records via our real-time API or test inbox placement with our inbox tester.

What happens when an SPF record exceeds the recursion depth limit?

When an SPF record in Microsoft 365 exceeds the 10-level recursion depth limit, the receiving server stops parsing at the 10th include directive and treats any further includes as invalid. This results in a neutral or permerror SPF result, both of which fail authentication and hurt sender reputation over time.

Why recursion depth matters in SPF validation

SPF records can reference other records using the include mechanism. Each include can itself contain additional includes, leading to a chain of references. Microsoft 365 enforces a hard limit of 10 levels deep. Once that threshold is reached, the server stops parsing and cannot validate the complete chain.

If your SPF record has more than 10 include levels—say, from nested third-party services, shared infrastructure, or misconfigured policies—the result is not a pass. Instead, you get a permerror if the parser detects a syntax issue, or neutral if it simply can’t verify the full chain. Both outcomes are considered failed authentication under industry standards.

According to the official SPF specification (RFC 7208), servers are allowed to reject or treat as neutral any record that exceeds their configured recursion limits. Microsoft’s implementation follows this standard strictly, meaning even short-term issues can trigger systemic rejection.

How that affects deliverability and reputation

A permerror or neutral SPF result doesn’t just fail the check—it signals to M365 that your authorization chain is broken or incomplete. Over time, repeated failures accumulate reputational penalties.

Even one or two per-recipient permerrors can cause your domain to be flagged as suspicious. This reduces inbox placement, increases spam filtering, and raises the odds your messages end up in junk folders. The more sends you make with failed SPF checks, the harder it becomes to recover sender reputation—especially when the root cause is buried in a deeply nested SPF chain.

Testing your domain’s SPF chain before sending can prevent this. Tools like MailTester’s inbox placement tester help simulate real-world delivery behavior, including SPF checks, across multiple inboxes.

For teams managing large lists, using an email list verification tool helps identify domains with problematic SPF records early—before they hurt your deliverability. You’ll catch misconfigured domains, catch-all addresses, and other red flags that might otherwise go unnoticed until after you've sent.

How to check if your SPF record exceeds the recursion depth limit

You can test for SPF recursion depth limits in Microsoft 365 by validating your SPF record using a tool like MxToolbox’s SPF validator or a compliant RFC 7208 parser. These tools walk through every include: mechanism, revealing hidden nesting chains. If your record contains repeated or deeply nested includes—especially from third-party services like SendGrid or AWS—it may hit Microsoft’s recursion limit (typically 10), causing email rejection even if the record is otherwise valid.

Step-by-step SPF recursion check

  1. Copy your domain’s full SPF record from DNS. Make sure to get the entire string, including all include: statements and modifiers.
  2. Paste it into a public SPF validator like MxToolbox’s SPF Checker or a tool compliant with RFC 7208. These tools analyze recursion and highlight every include: path.
  3. Look for any include: entries pointing to domains you don’t own—common ones are sendgrid.net, mailchimp.com, or amazon.com. Each one can trigger a new DNS lookup.
  4. Check for nesting. If one domain's SPF includes another, which in turn includes a third, recursion builds quickly. Tools will show this chain—sometimes through 10+ levels—without your knowledge.
  5. Count the total number of include: mechanisms, especially across nested domains. Microsoft 365 enforces a recursion depth limit of 10. Exceeding it means your SPF fails validation regardless of content.
  6. If your record exceeds the limit, consider switching to a single, authoritative include (e.g., include:_spf.yourprovider.com) or remove unnecessary includes. Simplify where possible.

Why recursion matters in email delivery

Even if your SPF record looks correct, deeply nested includes can break delivery in Microsoft 365. The receiving server stops after 10 levels and defaults to a fail, leading to bounce or spam placement. This is common when using multiple third-party services with complex SPF chains—especially in large-scale email campaigns.

Let's say you include Mailchimp, which includes Amazon SES, which includes another provider. That’s already three levels. Add more, and you exceed capacity. Even if you don’t see warnings in a basic validator, the depth can still break authentication.

To catch this before it impacts your sends, verify your full list using MailTester’s bulk verification. It checks both syntax and deliverability risks, including SPF and DMARC issues, before you send. You can also test sender reputation and inbox placement with MailTester’s inbox tester, which simulates Microsoft 365 and other inboxes to validate deliverability.

What is the impact of deeply nested SPF records in email verification tools?

Many email verification tools check SPF syntax but fail to simulate Microsoft 365’s real-world recursion limit of 10 include directives. A record that passes basic syntax checks may still fail in production due to this limit, causing legitimate emails to bounce. Tools that don’t test against actual M365 behavior give false confidence, risking sender reputation and deliverability.

Why syntax validation isn’t enough

SPF records can be syntactically valid but still break under real-world conditions. If a domain uses more than 10 include directives, M365 stops processing at the 11th, marking the email as invalid—even if the record looks correct on paper.

Let’s say a tool only checks for proper formatting: it’ll approve your SPF record as "valid" even if it nests too deeply. That’s a gap. You’re not just chasing syntax—you’re testing whether the email actually delivers in Microsoft 365’s environment.

How MailTester prevents real-world failures

MailTester’s verification engine doesn’t just scan for syntax. It simulates how M365 actually resolves SPF records during real-time validation. This means it detects recursion depth issues before they cause bounces.

For example, if your SPF includes ten third-party domains and a new one is added, MailTester flags it as risky—because the total exceeds the 10-include limit. This catch happens during bulk verification or API checks, so your list stays clean and your sender reputation safe.

This doesn’t happen with every tool. Some rely on static checks that don’t mimic real mailbox behavior. Microsoft’s own documentation confirms the recursion limit in its email processing stack (see Microsoft Learn).

If you’re managing outbound email at scale—whether for marketing, support, or transactional flows—knowing how M365 actually processes SPF is crucial. You can’t trust a tool that ignores this limit.

That’s why MailTester builds real-time M365 behavior into its verification workflow. Use our bulk verification tool to scrub your list, or integrate with our real-time API for ongoing validation. All checks reflect actual delivery behavior, not just theory.

How MailTester detects SPF recursion depth issues during verification

You don’t need to guess if an SPF record causes recursion issues in Microsoft 365 — MailTester checks it for you. Our real-time verification engine parses each SPF record exactly as Microsoft 365 does, tracing include chains to detect depth overage. If a chain exceeds 10 levels or includes nested third-party inclusions, we flag it as risky and show the exact path causing the problem, so you can fix it before sending.

Emulating Microsoft 365’s SPF parser behavior

MailTester doesn’t just scan SPF syntax — it simulates how Microsoft 365 actually processes includes during email validation. The engine recursively follows each include: directive, tracking every hop in the chain. This ensures we catch problems that would otherwise go unnoticed by tools relying only on basic syntax checks. The result? A true mirror of how your email will be evaluated in production.

Tracking include depth and reporting full paths

We measure the depth of include chains in real time, not just count them. If a chain reaches or exceeds 10 includes, or contains a third-party include embedded inside another, the domain receives a “risky” verdict. This is not just a warning — it’s a known trigger for Microsoft 365 to reject mail entirely. We don’t just say “problem detected.” We show you the full path, like include:spf.protection.example.com → include:thirdparty.verification.co → include:sub.include.net, so you know exactly where to adjust. This level of detail is rare and essential.

For teams running mass campaigns, this prevents unnecessary bounces and reputation damage. You can test entire lists in bulk or check individual addresses via our real-time API, with the same depth-aware parsing applied. Whether you’re using Mailchimp, Klaviyo, or SendGrid, you can integrate MailTester to validate SPF behavior before deployment. Learn how our integrations work with your stack to catch these issues at scale.

SPF recursion limits are defined in RFC 7208 — you can read the official specification at IETF RFC 7208 for the full details. That document states the requirement for a soft limit on includes, reinforcing why tools like MailTester must go beyond simple syntax and simulate actual processing behavior. It’s not just about compliance — it’s about ensuring mail actually arrives.

Best practices for maintaining SPF record compliance in Microsoft 365

You should limit your SPF record to no more than 10 include mechanisms to avoid hitting Microsoft 365’s recursion depth limit, which can cause authentication failures. Use a tool like an SPF record builder to track nesting and avoid excessive layers. Always validate your setup before deployment to prevent bounces and delivery issues.

Key Implementation Rules

  • Keep the total number of include mechanisms in your SPF record at or below 10. Microsoft 365 stops evaluating beyond this threshold, and exceeding it results in a soft fail on SPF alignment.
  • Do not include third-party domains unless you fully trust their infrastructure and control their email sending behavior. External includes increase complexity and risk.
  • Use a single, centralized SPF record—preferably managed at the domain level—rather than scattering include directives across multiple subdomains or services.
  • Only include domains you control or have explicit, written confirmation from trusted partners. Third-party providers should use their own SPF or be excluded entirely.

Alignment and Workarounds

  • When simplifying SPF records, rely on DKIM and DMARC alignment to maintain sender reputation and inbox placement. These protocols are more resilient to overly complex records.
  • Use DKIM signing with aligned domains to verify message origin even if SPF is relaxed. This gives you flexibility without sacrificing security.
  • Run a DMARC policy with rua and ruf reporting to monitor alignment failures and adjust your SPF or DKIM setup as needed.
  • Test your full email stack—SPF, DKIM, DMARC, and deliverability—before pushing to production. Use inbox placement tools to see real-world results across Gmail, Outlook, Yahoo, and other major inboxes.

Microsoft’s SPF evaluation has a hard cap on include recursion. Exceeding it means your message fails SPF verification even if the content is clean. This is why over-nesting is such a common cause of delivery breakdowns. The SPF specification explicitly limits the recursion depth to prevent denial-of-service through malformed records.

Instead of relying solely on SPF, you’re better off combining it with DKIM and DMARC. This layered approach allows simpler SPF records—sometimes just a single include or even no includes at all—while still maintaining strong authentication.

Use tools like MailTester’s inbox placement tester to validate your full authentication stack. Confirm that your messages pass SPF, DKIM, and DMARC checks across real email clients before sending to customers.

For bulk list hygiene or automated verification, mail verification helps catch invalid or risky addresses early. You can use the API to check individual addresses in real time, reduce bounces, and improve deliverability over time.

Common SPF include patterns that cause recursion depth overage

You’re hitting Microsoft 365’s SPF recursion depth limit when you nest include: mechanisms too deeply—especially across multiple senders, chained domains, or vendor includes that themselves pull in more includes. Microsoft allows up to 10 include levels, and exceeding this causes SPF validation to fail, leading to deliverability issues. Even a seemingly simple setup can break if one include points to another that includes a third, creating an unintended chain.

Red flag: Repeated or overlapping include directives

Using the same include:spf.example.com across multiple sender domains might seem harmless, but if those domains all reference shared third-party providers, the combined SPF record can quickly exceed the recursion limit. Each domain’s SPF is evaluated independently, so repeated includes accumulate across your ecosystem. This is common in organizations using multiple subdomains for sales, support, or marketing, each with their own include.

Chained includes and shared providers

When one domain’s SPF includes another that includes a third, and so on—like A → B → C → D—you risk creating a chain that exceeds Microsoft’s 10-level depth limit. This often happens with vendors like SendGrid, Mailgun, or Amazon SES, whose public SPF records may include multiple other domains. If you use include:sendgrid.net without checking its internal structure, you could be walking into a hidden chain. According to RFC 7208, the SPF specification defines a limit, but implementations vary—Microsoft limits includes to 10 levels, while others may allow more (see RFC 7208).

Even when multiple senders each have their own include: pointing to a shared provider, the combined chain can trigger overage. For example, if five departments each include include:vendor.com, and vendor.com itself includes include:cdn.something.net, the depth adds up quickly. This is especially common in large enterprises with decentralized marketing or support teams.

Prevention starts with visibility. Use tools like MailTester’s bulk verification to scan SPF records across sender domains before sending. You can also test sender reputation and inbox placement with our inbox tester, which checks how your email lands in real inboxes across providers, including Microsoft 365.

How MailTester helps verify SPF compliance in bulk lists for M365 sends

You can catch SPF include recursion issues before they cause M365 delivery failures by running bulk list verification with MailTester. It checks all domains in your list for SPF include depth limits, flags those with nested or excessive includes, and surfaces them as 'risky' so you can clean them before sending. This reduces bounce rates and protects sender reputation automatically.

SPF include depth checks at scale

Microsoft 365 enforces a recursion depth limit of 10 for SPF includes. If your sender domain or any domain in your list has more than 10 nested includes, M365 will reject the email. MailTester scans your entire list in real time, identifying any domain that exceeds this limit.

This isn’t a theoretical risk. The same limit is defined in RFC 7208, which governs SPF behavior across all major email providers. When includes reference other includes in a chain, it creates a loop that M365 will detect and reject—especially if it goes beyond the standard depth.

For example, if email1.com includes spf1.com, which then includes spf2.com, and so on, each chain is counted. MailTester traces this hierarchy across domains and reports the full path, so you know exactly where the risk lies.

Integrate and clean before sending

MailTester integrates directly with Mailchimp, HubSpot, Klaviyo, and SendGrid so you can plug SPF checks into your workflow—before a campaign launches. You can set up a pre-send validation that blocks any address flagged as 'risky'.

After verification, you’ll see each email’s status: valid, invalid, catch-all, or risky. Domains with excessive include nesting appear as 'risky' and can be filtered out or flagged for manual review.

Use the real-time verification API or bulk list check to validate large datasets in minutes. If you're still unsure, test inbox placement with MailTester's inbox tester tool to see how your messages land in real inboxes.

With 100 free verifications to start and credits that never expire, testing SPF compliance doesn’t cost anything upfront. Whether you're sending to 1,000 or 100,000 emails, MailTester helps you ship safely with M365.

See how it works: bulk verification, API, inbox placement, integrations, pricing.

Why real-time API verification with MailTester improves SPF testing accuracy

MailTester’s real-time API checks SPF records dynamically during each request, simulating Microsoft 365’s exact recursion depth limit. Unlike static tools that parse DNS records once and cache results, our API evaluates the current state of a domain’s DNS, catching failures caused by deep recursion that Microsoft 365 would reject. This means you avoid false positives—emails marked valid when they’d actually bounce in Outlook or Teams.

How SPF recursion depth impacts deliverability

SPF uses DNS lookups to validate sender domains. When a domain references other domains via include:, the resolver must follow those links. Microsoft 365 caps this at 10 levels of recursion. If a chain exceeds that limit, the SPF check fails—even if the chain is technically valid. Many third-party tools miss this because they stop at 5 or don’t simulate M365’s behavior at all.

Let’s say you’re sending to a domain with a long chain of includes. A static parser might say “valid,” but in reality, Microsoft 365 will reject the message due to recursion depth. That’s a lost delivery and a damaged sender reputation. MailTester’s API replicates that exact behavior in real time, so you know—not guess—whether an email will pass.

Why real-time evaluation beats cached data

Most email verifiers use pre-built databases or store DNS snapshots. That’s fine for static checks, but not for real-world validation. DNS records change. SPF chains shift. If your tool relies on outdated data, you’re sending to addresses that may not work.

MailTester’s API queries DNS at the moment of verification. No caching. No assumptions. Every check reflects the current state. This is why our accuracy is 98.9%—not because we guess, but because we test live. Competitors like ZeroBounce, NeverBounce, and Kickbox often rely on static parsing and can’t replicate M365’s limits with full fidelity.

For teams managing large sends, this precision saves time, reduces bounces, and protects domain reputation. You’re not just verifying syntax—you’re simulating the actual inbox environment. That’s why we built the real-time verification API as a direct line to deliverability performance.

Fixing SPF recursion depth issues: a realistic path forward

SPF recursion depth limits in Microsoft 365 can silently block valid emails. The root cause is often over-nested SPF records from multiple third-party vendors.

Auditing your domain’s SPF records and those of your vendors is the first step. Look for redundant or deeply nested includes. Consolidate these by using delegation via TXT records, or restructure with the 'redirect' mechanism to flatten the hierarchy.

Monitor your delivered send rates after changes to confirm inbox placement improves. Resend a test batch to your own email lists, and verify your full list using MailTester to catch any residual issues tied to SPF or other deliverability factors.

Sources

Keep reading

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

Frequently asked questions

What is the SPF include recursion depth limit in Microsoft 365?

Microsoft 365 enforces a maximum recursion depth of 10 include mechanisms. Any SPF record exceeding this limit fails during authentication.

Can SPF verification tools catch recursion depth issues?

Only tools that simulate M365’s parsing engine can reliably detect such issues. Many tools check syntax only, missing real-world failures.

How does MailTester verify SPF recursion depth in real time?

MailTester uses an active recursive parser that walks the full include chain, detecting depth overages during verification.

What happens if my SPF record exceeds the recursion limit?

M365 rejects the SPF check with a permerror, harming deliverability even if DKIM and DMARC pass.

Do I need to remove all third-party includes?

Not necessarily. But using more than 10 include mechanisms in total, especially nested ones, causes issues. Consolidate where possible.

How can I test SPF records without a live domain?

Use tools like MxToolbox or MailTester’s API to simulate parsing from a known recipient environment like M365.

Can a domain have multiple SPF records?

No – multiple SPF records cause authentication failure. All policies must be in a single TXT record.

Why do some SPF tools report my domain as valid when M365 rejects it?

They may verify only syntax, not recursion depth. MailTester checks real-world behavior, catching mismatches before sends.

Is the recursion limit the same across all email platforms?

No – some platforms allow higher limits, but M365 enforces strict 10-level recursion. Always validate against target recipients.

What is the best way to audit SPF records for large lists?

Use MailTester’s bulk verification with inbox-placement tests to identify high-risk domains before sending.