Eric,
 
How is    From:    A@B
          From:    X@Y
 
Different from
          From:    A@B, X@Y  (which is perfectly legal under RFC822) ?
 
As I understand it, if you have multiple FROM addresses there HAS to be
a SENDER tag which identifies the actual origin of the mailfile. The
sender tag has a higher "return priority" than FROM does anyway.
 
In other words, I would say that you can deal with the problem by
using the address specified in the SENDER field when multiple FROM
lines (or FROM addresses) are detected.
 
Andy