Yes, just about anyone can connect to sendmail on port 25. Whether or not they can send mail depends upon generally the IP address they are coming from (eg and approved for relay IP) and whether or not the mail is directed to something "on" that box (local delivery) or for something off that box (relay to another host/domain).
Thanks Larry - after I posted I thought about that. I told this user to not check mail over 30 minutes (our relay allowed time limit) and try this again; I didn't think about your other point, that mail within the box is not being relayed. And I knew that.
Thanks so much!!! Jale.