offlineimap and dovecot-antispam: APPEND issues

[Micah Anderson]

Hello,

I've recently implemented the dovecot-antispam plugin and was dismayed
to find that it has a particularly bad failure situation that only
occurs for Offlineimap users... of which I am one. This seems
unfortunate, and should be resolved. So I'm writing here to see what can
be done about this, and CC'ing the dovecot-antispam maintainer in the
process. Hopefully a good dialoge about the problem and potential ways
forward will result.

The summary of the problem is that offlineimap operates in a particular
way with regard to APPEND operations that make it impossible for the
dovecot-antispam plugin to detect certain spam training situations. 

The problem is when training filters. When a user is looking through
their Spam folder, and finds a message that was incorrectly tagged as
spam and decides to move it out of their Spam folder. The way
offlineimap handles this situation is that when it runs it does an
APPEND+DELETE, and this results in a situation that makes the
dovecot-antispam plugin not be able to detect that this is happening and
thus will not train the message as not-spam (ham). If the user does this
move in a webmail client, or other mail client, things work fine.

The dovecot-antispam man page has this warning to say about APPEND
operations, which is what offlineimap does:

ALLOWING APPENDS?

       You should be careful with allowing APPENDs to SPAM folders. The
       reason for possibly allowing it is to allow not-SPAM --> SPAM
       transitions to work with offlineimap. However, because with
       APPEND the plugin cannot know the source of the message, multiple
       bad scenarios can happen:

        1. SPAM --> SPAM transitions cannot be recognised and are trained

        2. the same holds for Trash --> SPAM transitions

       Additionally, because we cannot recognise SPAM --> not-SPAM
       transitions, training good messages will never work with APPEND.

I'm hoping that this is enough information for offlineimap developers to
understand what the issue is, but if not I've CC'd the dovecot-antispam
developer if further clarification is needed.

micah

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