Setup is this:

- There are local users, for which you want to greatly limit mail send rates
  (because they pick crummy passwords or have bad security on their home
  computers and their accounts get hijacked by spammers occasionally)

- The users can sometimes send mail as localhost (either via limited shell
  accounts or because webmail runs on the same host and uses local SMTP)

- Mailman runs on the same host, but does not get along with low rate limits at
  all

So, you want to say "mailman gets a free pass, but other connections from
localhost stay limited".

You can't restrict it by host, because both mailman and the users are coming
from the same place.

Solution:

In postfix, create a separate SMTP service just for mailman, and give it
"smtpd_client_event_limit_exceptions" (or unset the restrictions). i.e. in
master.cf:

588     inet  n       -       -       -       -       smtpd
  -o smtpd_client_event_limit_exceptions=127.0.0.0/8


Then, in mm_cfg.py for mailman:

SMTPHOST = 'localhost'
SMTPPORT = 588