Sendmail^TM is the most widely used Mail Transfer Agent. It is available in Open Source Form, as well as in the form of a commercial package. Sendmail^TM is a trademark of Sendmail, Inc.

Sendmail is often deprecated as either having poor security or being difficult to configure. My own opinion is that no open source MTA has had more attention in the area of security, and even though vulnerabilities may still exist in sendmail, they will most likely be fixed very quickly after discovery due to the many people using and maintaining this product.

As far as configuration goes, I have found that, at least on my RedHat Linux systems, I can do 90% of the configuration using the main sendmail configuration file, sendmail.mc. Recent versions of sendmail distributed in RedHat Package Manager (RPM) form support all of the features that I need, including encryption and filtering.

Here are some resources to help with the installation and maintenance of sendmail.: