I've just installed Mailman version 2.1.11.
I was surprised to find that the standard distribution had no support for the Postfix VERP feature, which can create VERP return paths much more efficiently than when Mailman does it itself.
I did find a patch for the FreeBSD port of an earlier Mailman version (at http://lists.freebsd.org/pipermail/freebsd-ports-bugs/2007-October/128777.ht...), and changed it to match the standard 2.1.11 distribution.
It seems to work fine.
If anybody should be interested, I've placed the modified patch, directly usable for 2.1.11, at http://www.dybdal.dk/mailman-2.1.11-postfix-verp.patch
Note that even with the patch, this feature is disabled by default; it is turned on by setting VERP_STYLE = 'Postfix'. It also needs to be allowed explicitly in the Postfix configuration (e.g., "smtpd_authorized_verp_clients=127.0.0.0/8").
I have tested the patch only with the feature turned on.
Jesper Dybdal, Denmark. http://www.dybdal.dk (in Danish).