I've written a milter to replace a hand-coded SMTP relay. This works,
but I'm having a problem handling bounces.
The 'relay' actually handles private mailing lists which must be
anonymised (for minors). A user sends a mail to 'virtual' addresses, and
the relay does a database lookup, and forwards the incoming mail on to
the real addresses of the recipients. 'Virtual' in this sense means that
all users get a made-up address ('[email protected]'), so the
relay just converts between real and made-up private addresses/./
In the Postfix equivalent, the new milter doesn't work without this
entry in virtual_alias:
@private.example.com my-email-address
Where 'my-email-address' is a local address (which isn't required or
monitored). So, Postfix first checks the incoming address in
virtual_alias; if there is a match, the milter is called. The milter
then anonymises and sends on the mail to the real recipient addresses.
Note that general name translation is /not/ handled in virtual_alias,
because there are hundreds of names, and they're constantly changing,
and this is all tied up with a membership database that does lots of
other things.
This all works, but there's a problem with bounces. If a bounce is
returned with RCPT TO set to <>, postfix/bounce takes over, looks up
virtual_alias, and does local delivery to 'my-email-address', /without
calling the milter/. This is no good, since I don't want to read all the
bounces myself (this was a real problem with the hand-coded relay). /I
want the bounce to go back to the original sender/. To do this,
postfix/bounce must pass the mail on to the milter instead of handling
it itself.
I have thought about deleting the 'bounce' line from master.cf (and
'defer' and 'trace'?). However, I can't do this, because Postfix also
handles 'normal' emails to '@example.com' (without the 'private'
subdomain). So, ideally, I want to keep postfix/bounce, but make it feed
on to the milter. Is this possible? Thanks.
