To try to find the problem, I have run sendmail using the -bv option on local user names, both with and without the domain name or IP address (user, user@domain, user@[ip.ad.dr.ess]). All return with the response "deliverable: mailer local, user user" - indicating that things should be fine.
I have run from the terminal prompt on the server with a "-v -C./sendmail.cf user@domain" set of options, and the mail was properly delivered.
However, any use of a mail program, either on the local network or from the internet, addressed to user@domain or user@ipaddress, gets bounced with a message to the effect of "user@domain is not a valid recipient" and hangs around the server's queue.
Unfortunately, I can't receive mail from LISTSERV's as a result of this (although I can send mail to them!), so I am hoping to find help on this Web forum.
Any advice would be appreciated.