dovecot - Oct 2009 - "From" address in vacation response on hosted domains with dovecot/postfix/sieve

Hi,

I have a working dovecot/postfix/sieve configuration:

Ubuntu 8.04 LTS
Dovecot 1.2.6
Sieve 0.1.12
Postfix 2.5.1

The server is configured to host multiple domains. All mail delivery, logins
and aliases work absolutely perfectly.

There's only one weirdness:  the 'From' address in vacation
responses.

If a message is sent to user at hosteddomain.com the response comes from
user-hosteddomain.com at hostingservername.com

Obviously it's doing what one would expect in that it's taking the
username
(user-hosteddomain.com) and appending the local host name
(hostingservername.com). This isn't great from the perspective of the hosted
user, however, who'd much rather their vacation message was sent from their
actual email address.

Clearly I can modify this by using a ':from' statement in each relevant
sieve file, but I can't rely on users' mail clients to set this (as not
all
do) and the webmail client in use on the server doesn't set this. I can
almost certainly implement some kind of hack using this, but I'd rather not.

So how does sieve determine the from address to use if it's not explicitly
set in the sieve script for the user, and how can I direct sieve to
determine the address differently, preferably with a central configuration
option that will apply to all users regardless of domain.....?

Regards,

Peter.