RE: [courier-users] Eliminating duplicate recipient addresses for local mail delivery

Chuck Williams <chuck@allthingslocal.com> wrote:
> I'm using default mail delivery to ./Maildir in each user's home
> directory, and I've got several distribution lists set up up as
> multiple-recipient aliases (defined in Webadmin). If a specific
> recipient occurs directly or indirectly in a message, that recpient
> recieves multiple copies of the same message. E.g., sending a message
> addressed like
>
> To: user1
> Cc: all
>
> where all is an alias that sends to all users (including user1), causes
> user1 to receive the message twice.
>
> Is there a way to configure Courier to consolidate recipient addresses
> so that in this case user1 would only receive the message once?

Note that it is not the various recipient headers but the envelope "RCPT
TO" recipient commands that are authoritative for where the message is
delivered.

Even if you looked at the envelope recipients, what you are trying to do
is generally impossible, because in the end it is not the delivered-to
_recipient_address_ that counts (as you already noticed) but the
delivered-to _mailbox_, which cannot be easily determined at an early
phase. For instance, take the following case:

RCPT TO: <user1@foo.org>
RCPT TO: <all@bar.org>

Even though <user1@foo.org> might be an alias to <user1@bar.org> (which
would be part of the <all@bar.org> distribution list), and both domains
might even be hosted on the same machine, Courier doesn't know until
delivery actually happens.

The best thing you can do is writing a Maildrop script that normalizes
messages (e.g. by simply taking the "Message-ID:" header for a very crude
implementation, or by removing all the "Return-Path:", "Received:", and
some other headers, and then computing an MD5 checksum of the remaining
message), keeps a database of delivered messages (in normalized form), and
drops duplicate messages for any given mailbox.



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