> I got several of these lately and don't understand them.        Jeff
 
Shoot the user's mail agent. Nothing you can do.
 
> Unable to deliver mail to some/all recipients.
> 050 RCPT TO:<all [log in to unmask]>
> 501 Syntax Error. '@' expected
 
You've probably got yourself a user on a SVR4 Unix like Solaris,
which ships a hopelessly broken version of BSD mailx that
requires commas between addresses instead of interpreting spaces
like the true BSD mailx did. The fix is to whack mailsurr on the
offending machine with a big magnet and get sendmail running with
a reasonable .cf file.
 
Unix=progress? Not bloody likely.