net-mail/mailman-2.1.8_rc1 uses the gid "qmail" when building with USE="qmail",
but qmail (as documented in qmail-lspawn) uses the uid/gid of the mailman user
when delivering mail to the mailman user (when delivering a mail to a mailing
list for example). This results in the following error in the qmail log:
@4000000044969fd023eb03d4 delivery 113: deferral:
Group_mismatch_error.__Mailman_expected_the_mail/wrapper_script_to_be_executed_as_group_"qmail",_but/the_system's_mail_server_executed_the_mail_script_as/group_"mailman".__Try_tweaking_the_mail_server_to_run_the/script_as_group_"qmail",_or_re-run_configure,_/providing_the_command_line_option_`--with-mail-gid=mailman'./
Replacing the gid qmail with mailman in the ebuild makes everything work as
expected.
Can't test it (no qmail around), so I need to trust you, but fixed it. Please
re-open if any problems occur due to this.