I've had the same problem before. In order for Mailman to work properly, it has to know the gid of the mail program at compile time. In this case it was told the mail program's gid was that of group "mailman", but when the script was executed it was with the gid of group "deamon". The easiest way I've found (and possibly the only?) to fix it is to use vigr to get the gid of the daemon group, then edit the mailman ebuild file. Right near the top of the file is an option for setting the mail-gid. Change that number to the one you found from vigr, then re-emerge mailman. Keep in mind that you'll have to repeat this procedure every time you upgrade mailman because the new ebuild won't have your changes in it.
Bryan
----- Transcript of session follows -----
Group mismatch error. Mailman expected the mail
wrapper script to be executed as group "mailman", but
the system's mail server executed the mail script as
group "daemon". Try tweaking the mail server to run the
script as group "mailman", or re-run configure, providing the command line option `--with-mail-gid=daemon'.
554 5.3.0 unknown mailer error 2
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