I'm receiving emails saying Underliverable Mail and it'ss from one of my web sites. I'm the only one with a mail account on this particular site, but yet these mails appear to be sent from strangename@mydomain.com. The Strange_Names change, but I get these messages daily. I need to secure my sendmail pgm.
version of sendmail loaded:
sendmail-cf-8.12.11-4.RHEL3.1.swsoft

I don't know what the swwsoft implies. It does not have a sendmail.mc file and I don't know how to create one from scratch.

What can I do to secure the site without stopping email for all the other domains hosted on the same server?

Do you know if the emails are actually being relayed through your server?

As anybody can fake the senders address, send out the SPAM and it then gets bounce back as non-deliverable to what the recipents mailserver believes to be the original sender.

Go to Google and type in mail relay test, this will give you a list of sites you can use to test your server.