Any time a message can't be delivered to a recipient's inbox when an unidentified (i.e. non subscriber) caller leaves a message, it will eventually end up in the unaddressed messages public distribution list. This can be due to a full mailbox or a down Exchange server after too many delivery attempts or the like.
If a subscriber leaves another subscriber a message that can't be delivered, it's simply bounced back to the subscriber that sent the message. In the case of an unidentified (i.e. outside) caller we have to fall back on this mechanism to make sure the message is not lost.
The message forwarded to you will actually be a forwarded email which can be opened from Outlook that will contain the original target user and message so you can forward it to the right person once the original problem is corrected.
Also, Unity is capable of being configured to not take messages from unidentified callers for subscribers that have mailboxes that are full - if this is what's causing your deliveries you may want to look at this - you didn't indicate which version of Unity you're using so I'm not sure if you have a version that supports this or not.
