I think I fixed my return-path issue with sqwebmail. It was a matter of configuring Exim4. See http://www.andreaswacker.com/blog/2006/08/11/return-path-exim4/. I simply added users and addresses to /etc/email-addresses.
Eg. john: email@example.com.
To UNSUBSCRIBE, email to debian-user-REQUEST@lists.debian.org
with a subject of "unsubscribe". Trouble? Contact firstname.lastname@example.org