I have successfully used fetchmail to pull messages via POP3 to post in the forum. The command-line syntax is shown below (it would have to be modified for use via cron or fetchmail running as a daemon as the mailbox password is prompted for).

fetchmail -p POP3 -u -m /path/to/femail_procmail_drush

In my setup, I have a single mailbox set up for "forums@mydomain.tld" with aliases for each of the forums that get delivered to the same mailbox. For example, if you have 2 forums "Forum Topic A" and "Forum Topic B", set up forum-topic-a@mydomain.tld and forum-topic-b@mydomain.tld to deliver mail to the forums@mydomain.tld mailbox, then have fetchmail retrieve messages from the forums@mydomain.tld mailbox.