# SMTP relay allows unauthenticated external delivery

An untrusted unauthenticated client can relay between external domains contrary to the server's relay policy.

**Candidate CWE:** [CWE-441](https://cwe.mitre.org/data/definitions/441.html)

## Test hypothesis

When mail delivery is explicitly covered, send one inert marked message from an owned sender domain to an owned external receiver through the tested relay. Use a source outside trusted networks and no inherited SMTP authentication.

## Evidence required

Source address, no-AUTH transcript, external envelope identities, queue response and receiver-confirmed delivery of the marker. Preserve the permitted submission comparison.

## Rule out false positives

Delivery to local domains, an allowlisted source, intended authenticated submission, RCPT acceptance without delivery, or a message routed through a different server.

## Playbook and command references

- [DNS, TLS and email trust boundaries](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/modules/dns-tls-mail.html) — Separate dangling records, transfer/recursion exposure, TLS identity and SMTP relay into controlled protocol tests.

[Find the associated command snippets](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/commands.html?q=DNS%2C%20TLS%20and%20email%20trust%20boundaries)

## Referenced by these target playbooks

[Network services](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/network.html)

## Report checklist

- [ ] Capture the affected version, configuration, identities, and object ownership.
- [ ] Preserve a valid baseline and a negative control.
- [ ] Reproduce the forbidden data access, action, or state change on controlled fixtures.
- [ ] Explain the intended security rule and why the observed behavior violates it.
- [ ] State prerequisites, uncertainty, cleanup, and the smallest demonstrated impact.

## Conditional ATT&CK mapping

No direct ATT&CK mapping selected. Record the weakness and demonstrated impact without forcing an adversary-behavior classification. See the [mapping guide](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/attack.html).

## Primary references

- [SMTP protocol](https://www.rfc-editor.org/rfc/rfc5321.html)
- [Swaks transaction controls](https://jetmore.org/john/code/swaks/latest/doc/ref.txt)
- [CWE-441 definition](https://cwe.mitre.org/data/definitions/441.html)

[Back to the complete vulnerability catalog](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/vulnerabilities.html)