When a regular user (peon) sends a private message to a usergroup, and another peon in the group has messages disabled from other peons disabled, it will still relay the message.

Detailed example:

Bob, Dale, and Sue are all members of the Active user group.
Bob has Private Messages from users disabled.
Sue is an administrator and is the group leader.
Dale sends a message to the Active user group, and both Bob and Sue will receive the message, despite Bob disabling the Private Messages.

------------

According to the forum's admin that this issue occurred on (I'm not an admin, moderator, or a server host, just a regular user), he recently upgraded to the latest phpBB3, which I'm sure is 3.0.2. I'm not sure if this is more of an intentional feature, rather than a bug, but I personally classify it as a bug due to this being abused to send spam.

I can provide more info at your request, and if I receive cookies and beer, a patch file (: