You said:
>>>> MAIL From:<[log in to unmask]>
><<< 502 No message receipt authorization.
>554 <[log in to unmask]>... Remote protocol error
 
Sounds like his incoming mail privileges have been cut off...or his
mailbox is full, or he's no longer working there, or any of a number
of other reasons why incoming mail would be refused.  If he's not
authorized to receive messages, you probably ought to NOMAIL him
at minimum.
 
Nathan