I got this one earlier this month.  Thought it was cute.
 
>binmail: /usr/spool/mail/userid: too big to accept new messages.
>        It's size is 205735 bytes which is 935 bytes over quota.
>mail: cannot open dead.letter
>554 <userid@host>... unknown mailer error 1
 
This is so wonderfully un*x....:))))
 
So I'm going to assume that 'unknown mailer error 1' means the user's
mailbox is full from now on...:)
 
Nathan