Our LISTSERV is running on LISTSERV.MYCORP.COM, but the "From:" field reads WWW.MYCORP.COM.

This is due to using an MTA that routinely "canonicalizes" the hostname of the local machine, and having LISTSERV.MYCORP.COM as a CNAME in your DNS. You can fix it by telling your MTA not to rewrite addresses (but be aware that this will work only until the mail reaches another MTA that isn't so configured), or by using a MX record instead of a CNAME, or by simply adding a second A record for the machine for LISTSERV.MYCORP.COM.

This was primarily a sendmail issue, but might still be a problem depending on the whims of mail transport agent authors and local mail administrators.