>         MAIL FROM:<[log in to unmask]>
>         501 Syntax Error. Unexpected Token '_'
 
This is an illegal RFC822 address. Underscores are not permitted
in domain names.
 
> The Listserv software seems to be objecting to the '_' in the gw_mail part of
> the address.
 
Correctly so.
 
> I have spoken to our network administrator about this, but he says he cannot
> change this easily.  Besides, he says, this is a legal groupwise address.
 
It may be "legal" to Groupwise, but it is not a legal RFC822
address. LISTSERV is conformant to the standards. SMTP is correct
in rejecting it.