Rules/MSGID MULTIPLE AT - Spamassassin Wiki

SpamAssassin Rule: MSGID_MULTIPLE_AT

Standard description: Message-ID contains multiple '@' characters

Explanation

The Message-Id: header contains more than one "@" characters, rendering it invalid. Invalid Message-Id headers have been seen generated by some types of spamming software.

Further Info

The default scores for this rule can be found in the online list of tests.

The syntax for the Message-Id field is defined in RFC 2822 sec 3.6.4, as well as recommended algorithms.

Bug #5707 suggests that Microsoft Office Outlook 12.0 (a.k.a Office Outlook 2007) generates invalid Message-Id fields, triggering this rule.

Note: Since dot-atom-text does not include the @ symbol, multiple instances usually indicate an invalid Message-Id. However it is possible for a syntactically valid (per RFC2822) Message-Id field to contain multiple "@" symbols under the circumstances that the id-left component consists of a double-quoted string (where qtext can contain %d64, allowing the "wrapping" of Message-Id fields) or inside id-right as part of a literal address string. For example:

Message-Id: <"12345@example.org"@host.example.com> 
Message-Id: <12345@[123@456]>
Message-Id: <"12345@example.org"@[123@456]>

This particular usage, however, is not currently covered by this rule and is not known to be in the wild.

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