[Dovecot] On-delivery deduplication?

[Tom Hendrikx]

On 08/06/11 22:48, Heiko Schlittermann wrote:
> Xin LI <delphij at delphij.net> (Wed Jun  8 22:04:51 2011):
> (…)
>> A feature of Cyrus-IMAPd I really missed after migrated to Dovecot is
>> their optional "duplicate suppression", which eliminates duplicate
>> message at deliver time, if their envelope sender, recipient and
>> message-id matches.  
> 
> Not sure, but I *think*, cyrus does dedup based on the message id and
> recipient only.
> 
>> For example, if one subscribes to a mailing list,
>> and someone hit "Reply All" to reply to him, there will be two different
>> SMTP deliveries, one from the replier's server and one from the mailing
>> list.
> 
> If your above statement about the dup detection is true, these two
> messages will be delivered both, since the envelope sender is likely to
> differ (most if not all list servers replace the envelope sender for
> several reasons (bounce detection, SPF passing, …))
> 
> 
>> Is it possible to do the same on latest Dovecot?  (Looking at the
>> documentation it seems not?)  Or is there some better ways to implement
>> the same functionality?
> 
> And these two messages are no duplicates. They differ in the headers, 
> probably they differ in parts of the body (signature of the list 
> server), EVEN they carry the same message id!
> 
> A duplicate is a duplicate if the message and its copies are identical.
> And from MPOV this is very unlikly to happen in common environments.
> 
> OTOH, if you need such feature, it shouln't be too challenging to write
> a MDA replacement, that decides about duplicity and finally passes the
> remaining messages to the Dovecot MDA.
> 

This happens because the person replying uses either the reply, or the
reply-to-all button in his MUA. The MUA should honour the headers of the
message it replies to, and most of the time MUAs do that.

People who want replies on their messages sent back only to the list,
and not to a personal address, should set the reply-to header of their
message to the list address:

From: me at example.com
To: list at example.org
Reply-to: list at example.org

This should always make a reply going only to list at example.org, whether
the person replying uses reply, reply-all or reply-to-list. This can be
done by hand, or with a mailclient plugins that does this for all
messages that are recognized as mailing list messages. Unfortunately I
did not find such a plugin for thunderbird yet...

NB I just set the headers in this message per my example above, for you
to test. If you hit any reply* button to react on to this message, a
decent MUA should not include my personal address as a recipient of your
message.

-- 
Regards,
	Tom