Recieved mail from email@example.com.
It was infected with the KLEZ virus.
This puppy grabs your address book and sends itself to everybody there.
Then you start getting "Returned Mail" to your mailbox.
Before you know it...your "Mailbox is full".
Both Symantec and MacAfee have free downloads to clear this up.
I'm taking them home tonight and running them through my laptop.
I did get a "xxx.btl" mailbox full return so I suspect someone on they're net has the bug.