We are trying to send email through a Virtualmin Pro server, using clients like Outlook.

We are unable to authenticate against the SMTP server.
We have tried plain text and ssl.

Whenever you try to authenticate, are you getting an error message on the client?

Also, what shows up in the log files on the server as you do that? Which file you look in depends on the distribution you're running, but for Debian/Ubuntu you might look in /var/log/auth.log, and RHEL/CentOS you might see some problems in /var/log/secure and /var/log/maillog.

And just to be sure -- you have port 25 opened up if you're running a firewall on your server?
-Eric

We are not getting an error on the Outlook side it just keeps asking for the user name and password.

Port 25 is open, we have verified that.

250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-AUTH PLAIN LOGIN
250-AUTH=PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN

Selecting SSl on Outlook gives the same results.

Rick

Solved:
http://www.virtualmin.com/forums/help-home-for-newbies/re:unable-to-send-mail-with-postfix-use-mail- clien.html