The forum itself, doesn't "hold" any mail. Delivery delays are typically associated with problems with the sending smtp server, with the route the mail takes, with the destination pop server, or with spam scanning on the pop server.
I have my smtp server set up to route all undeliverable email messages to a special mailbox. It will attempt to deliver again every 12 hours for four days before giving up. Examination of the undelivered messages indicates that the overwhelming majority of delays are due to problems with...
1. Invalid Email Addresses
2. Rejection by the pop server/email client as spam.
3. Pop server is down.
Yahoo, MSN, and AOL are notorious for upgrading their spam catchers and blocking valid email. Usually takes them a couple of days to get things straightened out after an upgrade.
Spam catchers in general have a vigilante attitude - block first and ask questions later. Often they block a range of ip addresses used by a shared hosting service because of reports that spam has been received from within that range. It takes the hosting service a couple of days to get it appealed and straightened out. until then, all mail is stopped by the spam catcher. This is the main reason most isp's do not allow maillists.