problem in Manmail Makefile?

[Geo]

Mailman  Makefile contains:
 ---------------------------------- 
 .if defined(WITH_SENDMAIL)
 MAIL_GID?=       mailnull
 .endif
 ------------------------------------ 
 But after installation (FreeBSD 6.2, /usr/ports/mail/mailman) when I send
 any message
 to mailman list I receive the error:
 -----------------------------------
Group mismatch error.  Mailman expected the mailwrapper script to be
 executed as group "mailman", but the system's mail server executed the mail
 script as group "mailnull".  Try tweaking the mail server to run the script
 as group "mailman", or re-run configure, providing the command line option
 `--with-mail-gid=mailnull'.
--------------

 What (and how!) can I do to set MAIL_GID= mailnull?

 If I understand I'll describe this in detail for users!