SendmailTM 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. SendmailTM 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