If LISTSERV receives mail that has a tortured enough mail header,
 it will send insert new header line with the e-mail address of
 the local postmaster.  The name field of that line will be set to
 "... c/o postmaster" so that people will know it wasn't really from
 the postmaster.  If you fetch the mail file in question from the
 archives, there should be some indication that the original "From:"
 field was invalid.
 
 -jj
 
 PS - Here's an example from one of JHUVM's list archives.  Nothing
 like a concrete example to explain something... :)
 
 ------ sample mail file follows:
> print all of 1308
>>> Item number 1308, dated 92/02/02 11:40:26 -- ALL
Date:         Sun, 2 Feb 1992 11:40:26 EST
Reply-To:     Multi-byte Code Issues <[log in to unmask]>
Sender:       Multi-byte Code Issues <[log in to unmask]>
Comments:     <Parser> E: Mail origin cannot be determined.
Comments:     <Parser> E: Original tag was -> FROM:
              "xxxxxxx%nlsarch.austin.ibm.com"
              <xxxxxxx%nlsarch.austin.ibm.com%SUN.C
From:         Undetermined origin c/o Postmaster <[log in to unmask]>