I seem to need to refer to this more often than I would have imagined: Postfix Mail Queue Management.
So I found this article which said:
$ sudo -u postgres psql
postgres=> alter user postgres password 'apassword'; postgres=> create user your-user createdb createuser password 'passwd'; postgres=> create database your-db-name owner your-user; postgres=> \q
Note: to enable password logins for the ‘postgres’ admin account, edit: /etc/postgresql/9.4/main/pg_hba.conf and after this line:
local all postgres peer
Add this line:
local all postgres md5
I found the Site Administrator Documentation for Mailman. In it I found the /usr/lib/mailman/bin/mmsitepass command which can reset the site password.