[Mimedefang] Return-Path help

[Mark Johnson]

I know this isn't the right list for this, but I thought maybe I could 
use MimeDefang to fix a real minor issue I'm having.  I'm using Cyrus 
IMAP server and sieve for filtering.  If someone set's up a vacation 
message, sieve autoresponds but sets "Return-Path: <>".  I'm noticing 
that many other autoresponders do the exact same thing, but more and 
more often, I'm noticing that customers mail servers are not accepting 
those messages with this:

   ----- The following addresses had permanent fatal errors -----
<someuser at domain.com>
    (reason: 556 Mail from nullsender not allowed (#5.7.1))

I can't figure out how to make sieve add the correct name in the return 
path, so is there a way to make Mimedefang do it on the way out?  Set 
the return-path to whatever the from has in it?  And should I even 
attempt this?  I've been reading and it looks to me like this was maybe 
done on purpose to prevent mail loops, and mail servers are supposed to 
accept messages with null Return-Paths.

Anyone have any ideas on this?  Thanks for the input!!

Mark