How should organisations manage SPF includes when multiple third-party services send mail for the same domain?

What SPF includes are doing in a multi-sender mail setup

SPF only works when the domain owner keeps the published record aligned with the systems actually sending mail. With several third-party platforms, the SPF record becomes a living allowlist for outbound mail, not a static configuration. The practical question is less about syntax and more about whether each sender is still authorised, whether its include path is valid, and whether the total DNS lookups stay within the limit.

That is why a good SPF design starts with sender inventory. Every service that can emit mail for the domain should be identified, mapped to the domain or subdomain it uses, and checked for its own SPF publication. When a provider uses nested includes, the organisation still owns the final record and should understand the full lookup chain before changes are approved.

For broader email-authentication hygiene, the same governance posture belongs in the DMARC, SPF and DKIM Guide, which covers SPF lookup limits, flattening and rollout discipline. The key point is that SPF is only dependable when the list of senders matches reality, including vendors that are added, changed or retired over time.

How to keep SPF includes from becoming brittle

The safest operating model is to treat each include as a dependency with an owner, a business justification and a review date. If the service is no longer sending mail, remove it. If the vendor changes its SPF structure, retest the record before the change is deployed. If a new platform is added, check its include chain against the existing DNS budget before it is permitted to send on the domain’s behalf.

Nested includes are where teams usually lose control. A record can look short and tidy while hiding multiple extra lookups behind vendor-managed includes. That can break SPF evaluation or create hidden fragility when a provider updates its own record. Organisations should therefore validate the full resolved path, not just the top-level SPF string, and keep a record of which vendor depends on which include.

In practice, this is the same kind of lifecycle discipline described in the IAM and IGA Basics guide, where ownership, review and entitlement control are central to keeping access accurate over time. The same logic applies here: an approved sender list is only trustworthy when changes are reviewed, not assumed to remain valid.

What to watch before and after a change

Before deployment, validate the published SPF record in a test path, confirm the vendor can actually authenticate mail for the domain, and ensure the include count still leaves room for future services. After deployment, monitor mail flow and authentication results so that a failed lookup, deprecated sender or vendor-side change is caught quickly.

In complex estates, SPF problems often show up as gradual drift rather than a single break. One team adds a helpdesk platform, another adds marketing automation, and a third enables a SaaS notification tool, but nobody revisits the combined DNS cost. Over time, the domain can exceed lookup limits or carry inactive includes that no longer serve any purpose. The result is not just a failed SPF check, but a weaker trust posture for the domain as a whole.

For organisations that want a structured external benchmark, the OWASP Non-Human Identity Top 10 is useful as a reminder that machine and service credentials need continuous governance, and the same principle applies to mail-sending services that act on the domain’s behalf.

Risk and Threat Considerations

SPF include sprawl creates both reliability and abuse risk. If an unused vendor remains authorised, it enlarges the trusted sending surface for no operational gain. If a third-party service is compromised or misconfigured, the attacker may gain a legitimate path to send mail that appears to come from the domain. Lookup-limit failures also create a quiet control weakness, because a record that cannot be evaluated correctly can undermine mail authentication at scale.

Failure mechanism: Nested or stale includes push the SPF record past DNS lookup limits, leave abandoned senders authorised, or preserve trust in a vendor whose mail path has changed or been compromised.

Impact: Legitimate mail may fail authentication, while malicious or unauthorised mail can inherit the domain’s credibility, increasing spoofing, phishing and delivery problems.

Practitioner Guidance

What to prioritise: Maintain a current sender inventory first, then compare it with the published SPF record. If you cannot point to the business owner for a sender, or explain why it still needs authorisation, it should be treated as a removal candidate.

What to verify: Confirm the full resolved SPF chain, not only the visible top-level include. Check the lookup count after every vendor change, because a vendor update can break your domain even when your own DNS has not changed.

Common mistake: Teams often optimise for brevity by piling on includes and assuming provider-managed SPF will stay stable. That is a governance error, not a convenience, because the domain owner still owns the final trust boundary.

Practitioner takeaway: The right operating model is continuous sender governance, if the service is not actively sending mail, or if its SPF path cannot be validated end to end, remove or fix it before the domain depends on it.

Related resources from NHI Mgmt Group

• How should security teams handle SPF management when multiple cloud and SaaS services send email for the same domain?
• Should organisations give third-party identities the same governance as employee accounts?
• How should organisations manage third-party access as part of IAM governance?
• What should organisations do when third-party AI services change unexpectedly?