In the line of "Which operating system, which browser, and which distribution of Linux should I use?" is the question, "Which Email server should I use?" While there are no solid answers hopefully I can shine some more light on your options.
I always believe in using the right tool for the right job, so while there may be some instances to trade security for speed, you better know what you are doing. For everyone else finding that sweet spot between security, speed, and flexibility can be difficult. There are many different MTAs (Mail Transfer Agent) available, some very good and some very bad. We will be looking for MTAs that are secure and easy to administer. After all most people just want to get their email working and not have to think much about it.
For the longest time the most popular MTA was Sendmail, but with an obscure configuration file and a long history of security breaches allowed other faster, more flexible, and secure MTAs to come to light. The two I've chosen to focus on are Postfix and Exim.
Postfix (Short Answer : Secure, easy to administer, efficient.)
Postfix is, a drop-in replacement for Sendmail. Postfix has a monolithic main configuration file with an interface like most other Unix programs and was written with security in mind. The author specializes in writing software that has been proven harder to break. As secure as the software is it remains quite flexible and manageable in its configuration, but not to the extent of Exim which we will see in a bit.
However that security design goal prohibits some very convenient features Exim offers. Postfix is a great balance of speed, flexibility and ease of administration but still remains highly secure. If you have good reason to need high security, then depending on how big your network is, you would probably consider using Postfix for Internet-facing servers and some other mta for local mail.
But, if your network isn't that big and security is paramount, Postfix is the way to go. Postfix is compatible with Sendmail command line interface, and modules.
Finally when you do have problems or want to go to the next level with Postfix. The Postfix community is very active. There are lots of resources for postfix documentation and support. Postfix comes installed by default with Plesk.
Exim (Short Answer: The all-purpose MTA. Much more configurable)
By design Exim was intended to be an all-purpose MTA. Exim doesn't restrict its feature set in order to achieve theoretical security, like Postfix. Exim give administrators the reliability and performance they want, with a strong bit of security. Although Exim can't ever be as secure as Postfix, it seems to be secure enough for most admins and day to day use.
Exim is also a Sendmail drop-in replacement, can deal well with high loads, has had good security record over the last seven years and can be extended in many ways.
Exim can become all-purpose or special-purpose by adding or omitting features during compile-time. Exim is also well documented and has an active community. Exim is available by default in WHM/Cpanel.
I don't know, but maybe now you do.