In 3.1(5) there is an option for us to check the target mailbox being full (level 2 full - not recieving messages) and we'll tell the outside caller trying to leave a message that the mailbox is full and we can't take their message now. The reason the check is optional for outside callers is because trucking across the network and checking the current mailbox status across the network can result in a small delay - some sites may find that unacceptable.
For sub to sub messages, however, the message is always send and then will NDR back to the sender (i.e. like if you send a message using Outlook).
for earlier versions of Unity, outside caller messages that can't be delivered for whatever reason (including full mailboxes) end up getting delivered to the Unaddressed Messages public distribution list created by Unity setup. You should have at least one admin type that gets these messages and make sure they end up in the right place.
