Tracing SPF Include Chains Behind Suspicious Email Senders
A suspicious email can appear to come from a familiar brand while its sending infrastructure is controlled by a completely different organisation. SPF analysis helps investigators examine the authorised mail servers behind an envelope sender and identify relationships that are hidden from the visible From address.
An SPF record is a DNS TXT record that lists permitted sending sources for a domain. Its mechanisms may point to other domains through include, redirect, and related directives, creating a chain of providers, platforms, resellers, and customer-specific services.
The chain is useful evidence, but it must be interpreted carefully. SPF authenticates the return-path or envelope-from domain, not necessarily the address displayed in a mail client. A reliable investigation combines SPF with DKIM, DMARC, headers, domain age, reputation, and the sender’s business context.
Start with the envelope sender
First, inspect the full message headers and locate the Return-Path, SMTP MAIL FROM, and Received-SPF fields. The visible From address may use billing.example.com, while the actual SPF identity belongs to a completely unrelated marketing or transactional mail domain.
This distinction is especially important when reviewing scam messages reported by customers in Sydney or Melbourne. A fake invoice may imitate an Australian bank’s branding while using a newly registered domain for the envelope sender. Extract the exact domain, preserve the original headers, and avoid relying on screenshots or copied message text.
Retrieve the complete SPF record
Query the sender domain’s TXT records using a DNS tool such as dig, nslookup, or a reputable online resolver. Look for a record beginning with v=spf1. If several SPF records exist, the domain is misconfigured; receiving systems may return a permanent error rather than evaluating them consistently.
Record every mechanism in order. Common entries include ip4, ip6, a, mx, include, and redirect. A mechanism such as include:mailer.example.net instructs the receiver to evaluate that second domain’s SPF policy. It does not mean the first domain owns the infrastructure, so the relationship needs further investigation.
Follow every include and redirect
Resolve each include recursively until the chain ends in IP addresses, an empty policy, or another reference. Keep a tree of parent domains, child domains, record contents, and lookup depth. A provider may include several regional domains, which can expose separate infrastructure used for Australia, New Zealand, or Asia-Pacific traffic.
SPF has a limit of 10 DNS-causing lookups per evaluation. include, a, mx, exists, and redirect can consume this budget, while ip4 and ip6 entries do not. Long chains that approach the limit are operationally fragile and can generate permerror, even when the sender is legitimate.
Separate infrastructure from ownership
An include chain often reveals a cloud email platform, customer relationship management system, helpdesk, or bulk-mail provider. Compare the organisations behind each hostname using WHOIS history, TLS certificates, passive DNS, autonomous system data, and reverse DNS. A branded SPF record that ultimately authorises a shared provider is common and not automatically malicious.
| Finding | Possible meaning | Investigation value |
|---|---|---|
include points to a major mail provider |
Legitimate outsourced sending | Moderate; verify alignment and headers |
| Several unrelated providers appear | Complex business stack or poor governance | Moderate to high |
| Newly registered include domain | Disposable or compromised infrastructure | High |
Broad ip4 range in an unfamiliar ASN |
Over-permissioned or suspicious authorisation | High |
redirect= leads to a different brand |
Managed policy or hidden dependency | High |
~all or ?all at the end |
Soft or neutral enforcement | Contextual |
Treat the chain as a map of authorised relationships rather than proof of the sender’s identity. The Trusted Sender Score team provides background on a platform designed to help domain owners and security teams assess this wider trust picture.
Look for suspicious chain patterns
Pay attention to sudden changes in the chain, especially a newly added include, a domain with no clear corporate connection, or an IP range associated with hosting rather than email delivery. Attackers may compromise a legitimate service account, exploit a forgotten subdomain, or register a lookalike provider name.
Other warning signs include circular references, excessive nesting, duplicate records, wildcard DNS, and a final -all policy that does not match actual delivery behaviour. A sender using Australian branding but routing through infrastructure in an unexpected jurisdiction is not automatically fraudulent, though it deserves corroboration against the organisation’s published security contacts and normal sending patterns.
Correlate SPF with DKIM and DMARC
Check whether the message contains a valid DKIM signature and whether its d= domain aligns with the visible From domain. Then inspect the domain’s DMARC record and determine whether SPF or DKIM alignment passed. A valid SPF result for an unrelated envelope domain does not make a spoofed visible From address trustworthy.
For repeat investigations, use anti-spoofing guidance alongside header analysis. This is particularly useful for Australian organisations handling payment requests, Medicare-themed scams, or parcel notifications, where brand impersonation is common and users may be working from distributed offices across Brisbane, Perth, and regional areas.
Turn the chain into an incident record
Document the original message hash, timestamps, envelope sender, visible From address, SPF result, every DNS response, resolved IP address, ASN, and relevant reputation findings. DNS changes quickly, so record the query time and preserve copies of the records where policy permits.
A bulk checker can help compare related domains, while an API can add sender verification to mail gateways, ticketing systems, or fraud workflows. A practical sender score walkthrough can support repeatable checks before analysts escalate a domain to a provider, registrar, or internal incident-response team. This evidence-led process distinguishes a complicated but legitimate mail setup from infrastructure deliberately assembled to conceal a phishing operation.