I am using postfix as my MTA. It is running on ubuntu and I am administering the system through webmin, try as I might with different configurations, I can't send email from a client. when I look in the mail.log i only find:

Jul 26 15:31:00 alpha postfix/smtpd[17430]: connect from unknown[99.153.1.214]
Jul 26 15:31:31 alpha postfix/smtpd[17430]: lost connection after UNKNOWN from unknown[99.153.1.214]
Jul 26 15:31:31 alpha postfix/smtpd[17430]: disconnect from unknown[99.153.1.214]

after trying to send a message. I have no idea what could be wrong.

thanks for any help.

Have you tried using this document?

http://www.postfix.org/BASIC_CONFIGURATION_README.html

You should install Postfix and make just the minimum changes outlined here. That will help you to understand how it works.