Good, "simple" question; complex/hard answer.
 
Yes, no, maybe.
 
I'm not quiet ready to create another e-mail on
the complexities and dis-synchronization of the
domain name system.  Suffice it to say though that
when users move to another e-mail system (or host)
that their userids don't necessarily stay the
same.
 
It sometimes helps to check users' previous posts
to your list (archives) and scan for .signature block
that give clues.
 
Sometimes the NODMGT-L archives have references to
Bitnet-to-Internet addresses.
 
A note to POSTMASTER or ROOT or SYSTEM at the
host can help.
 
Using DiG or NSLOOKUP against the SOA record in the DNS.
 
FINALLY, using the InterNIC WHOIS db for a technical
contact for the domain.
 
Been there, done that (the above).
 
--  co-owner:   INFOSYS, TQM-L, CPARK-L, ERAPPA-L, JANITORS, LDBASE-L, et -L
URL:mailto:[log in to unmask]                       "BEEP if you luv LDBASE"