How to spot a suspicious SPF include in your domain

An SPF record tells receiving mail servers which systems are authorised to send messages for a domain. Its include mechanism delegates that permission to another domain, commonly a marketing platform, helpdesk, payroll provider or customer relationship management system.

That delegation is useful, but it also expands your email attack surface. An unauthorised include added after a DNS account compromise can permit an attacker’s infrastructure, or a compromised third-party service, to pass SPF checks. Distinguishing a legitimate service’s SPF include from a malicious addition requires technical evidence rather than familiarity with the provider’s brand name.

This matters for Australian organisations sending invoices, appointment reminders and account notices from domains used in Sydney, Melbourne, Brisbane or regional offices. A sound review supports obligations under the Privacy Act 1988 and helps reduce phishing exposure under the Spam Act 2003, particularly when email is part of customer or government-facing operations.

Understand what an SPF include authorises

An SPF record is published as a TXT record, such as v=spf1 include:mailer.example -all. The included domain publishes its own authorised IP addresses and mechanisms. The receiving server evaluates those details when checking whether a message came from an approved sending system.

The include does not prove that every message is genuine, nor does it authenticate the visible display name. SPF generally validates the envelope-from or return-path domain. DMARC then connects that result with the visible From domain through alignment, while DKIM can provide a cryptographic signature that survives some forwarding scenarios.

An include is therefore legitimate only when your organisation has an active relationship with the named provider and the provider documents that exact SPF domain. A familiar company name in an include is insufficient evidence, because attackers may use lookalike domains or exploit an abandoned vendor relationship.

Build an evidence trail before changing DNS

Start with the current TXT record, its publication history and your organisation’s supplier register. Compare the include against contracts, onboarding records, security questionnaires and vendor documentation. Ask whether the provider still sends mail for your domain, which products use it and when the relationship was approved.

Check the included domain itself. Follow nested includes, a, mx, ip4 and ip6 mechanisms, and note every resulting address. A reputable provider usually maintains a stable, clearly branded policy record and publishes support guidance. A recently registered domain, unexplained redirect, disposable hosting range or unrelated geographic infrastructure deserves escalation.

For supplier-heavy environments, bulk domain checking can reveal inconsistent SPF, DKIM and DMARC controls across vendors. This is useful for Australian retailers, universities and healthcare groups that manage dozens of sending partners.

Look for signs of an unauthorised change

Compare the current SPF value with a known-good baseline from DNS monitoring, source-control records or archived security reviews. Warning signs include a new include that appeared without a change ticket, a vendor no one recognises, a sudden increase in DNS lookups, and a switch from a strict -all ending to a softer ~all or ?all.

A suspicious include can also be hidden inside a trusted-looking chain. An approved email platform may itself reference another service, but that relationship should be documented and technically necessary. Review the full resolution path rather than stopping at the first familiar domain.

Investigate DNS provider audit logs, registrar access, identity-provider alerts and administrator sign-ins. If the record changed unexpectedly, preserve evidence before editing it. A compromised DNS account can be used to alter MX, DKIM, DMARC and website records as well as SPF.

Validate the provider and the sending behaviour

Contact the vendor through a known channel from your contract or official website, not through details contained in an unexpected email. Confirm the exact include hostname, authorised products, sending regions and whether the provider recently changed infrastructure. The answer should match the published SPF policy and observed mail headers.

Examine legitimate messages in Australia’s normal business context: invoices from a Melbourne supplier, booking notices for a Brisbane clinic or delivery updates sent to customers in Perth. Inspect Return-Path, Received, Authentication-Results, DKIM signing domains and DMARC alignment. A message can pass SPF through a valid provider while still failing DMARC or carrying suspicious content.

A bulk trust audit can help security teams compare sender reputation and authentication results across multiple domains. It is particularly valuable after a merger, marketing-platform migration or change in an outsourced payroll provider.

Avoid breaking legitimate mail while containing risk

Do not delete an uncertain include without identifying which systems depend on it. Removing a valid mechanism can cause invoices, support tickets or one-time login codes to fail SPF, creating deliverability problems for customers and staff. First determine whether the vendor can use DKIM signing with your domain, a dedicated return-path or a narrower IP-based authorisation.

If compromise is plausible, temporarily isolate the affected service, rotate DNS and registrar credentials, revoke unauthorised sessions, and contact the provider’s security team. Preserve the old record and timestamps. A controlled replacement is safer than repeatedly editing SPF while the attacker may still control the account.

Keep the record within the SPF limit of ten DNS-querying mechanisms. Excessive nested includes can produce a permerror, causing receivers to treat authentication as unreliable. Consolidation should be performed carefully, because flattening provider IPs can become stale when their infrastructure changes.

Monitor authentication as an ongoing control

SPF review should sit alongside DKIM key rotation, DMARC reporting and domain ownership checks. Set DMARC reporting to collect evidence, then examine authentication results for unexpected sources, alignment failures and sending patterns that do not match approved business activity.

Schedule reviews after supplier changes and at regular intervals. The email authentication checklist can support repeatable checks across operational and marketing domains, including those used by Australian subsidiaries and agencies.

Record each approved include with its business owner, purpose, vendor contact, review date and replacement plan. When a contract ends, remove its include and associated DKIM selectors promptly. A small, documented SPF policy is easier to investigate, less likely to exceed lookup limits and harder for an attacker to hide inside.