Low Severity

Headers: Reserved domain in Message-ID

Labels

Spam
BEC/Fraud
Spoofing
Evasion
Header analysis

Description

Detects inbound messages where the Message-ID header contains a reserved or placeholder domain (such as example.com/net/org or a subdomain ending in .test, .invalid, .example, or .localhost). These domains are set aside by RFC standards for documentation and testing purposes and should never appear in legitimate production mail headers, making their presence a strong indicator of spoofed or synthetically generated messages.

References

No references.

Sublime Security
Created Oct 26th, 2026 • Last updated Oct 26th, 2026
Feed Source
Sublime Core Feed
Source
GitHub
type.inbound
and regex.icontains(headers.message_id,
                    '@(?:example\.(?:com|net|org)|[^@]*\.(?:test|invalid|example))>?$'
)
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