SpamAssassin rejects anonymized messages originally posted by Mozilla

[ikedas]

Version

Maybe any of 6.2.66 and earlier.

Installation method

Maybe any.

Expected behavior

Anonymized messages (by anonymous_sender parameter enabled) should not be rejected by content filters.

Actual behavior

SpamAssassin rejects the message with following status:

X-Spam-Status: No, score=0.313 required=6.31 tests=[AWL=0.643, BAYES_00=-1.9,
 FORGED_MUA_MOZILLA=2.309, HEADER_FROM_DIFFERENT_DOMAINS=0.25, 
 HTML_IMAGE_RATIO_02=0.001, HTML_MESSAGE=0.001, MAILING_LIST_MULTI=-1, 
 NO_RELAYS=-0.001, T_KAM_HTML_FONT_INVALID=0.01] autolearn=no autolearn_force=no

The message is rejected mainly due to FORGED_MUA_MOZILLA rule. It means that, though the message seems posted with Mozilla Thunderbird, the format of Message-ID field looks unfamiliar.

Additional information

Originally reported on a thread in sympa-users:
https://listes.renater.fr/sympa/arcsearch_id/sympa-users/2021-12/2b094340-6b60-4254-7030-cd1621385f1d%40prompt.nl

[ikedas]

Possible workaround is adding User-Agent to anonymous_header_fields parameter, because SpamAssassin uses this field to determine Mozilla Thunderbird.

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In this occasion, it may be pointed out that the uniqueness of the message ID in anonymized message is not ensured: Currently the string <listname.xsequence@anonymous> is generated. cf. related I-D (here and here) for encouraged form.

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EDIT: Added link to another I-D.