This is a very specific case, but:

- postfix is configured to have a special port dedicated to mailman, which
  bypasses normal delivery restrictions

- the normal smtp port works fine for delivery

- the mailman port doesn't, and complains of errors like:

postfix/smtpd[]: NOQUEUE: reject: RCPT from unknown[]: 451 4.3.0 <>: Temporary lookup failure; from=<> to ...

There are a lot of these, because a mailing list post goes to lots of
addresses, which can hide the fact that this line might also be present

postfix/smtpd[]: warning: connect to mysql server unix:/var/run/mysqld/mysqld.sock: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

...This line is caused by postfix being set to chroot (which it does by default
in, unless you tell it not to, and which it can do per service in -- e.g. normal smtp can be not-chrooted [which explains why it
works], but you left the special port with default settings), so
/var/run/mysqld doesn't exist in the chrooted environment. The solutions are:

- Don't chroot


- Connect to mysql with TCP/IP instead of socket