> I have never understood why XMAILER aborts the entire delivery if a
> single recipient is misspelled.
 
Who says it does that?  From my observations, XMAILER delivers to all
the valid recipients, regardless of an error in one.  A rejection
does get sent saying "some or all" could not be delivered, but the
BSMTP transaction log has specific error messages for those recipients
that were found to be invalid.
                                          John