Server :: How To Set Sendmail To Allow Authenticated Relay?

I have a CentOS server, and am using Sendmail to relay for various domains. However, I also need to have users send their outgoing mail via this server. I want to have them enter a username and password in Outlook, Thunderbird, etc in their smtp server settings. I currently have this set up via an Exchange server, but I cannot get it to work with Sendmail. I know little about Sendmail, and have done all the config via Webmin, which has worked fine as far as relaying domains. But even though I have added the CentOS user account for the smtp relaying, the clients mail apps give an error saying relaying denied.