Ranked in these QuestionsQuestion Ranking
Pro Flexible and easy to administer
Postfix uses a monolithic, flexible, human-readable configuration file with sane defaults and since it's modular, each module can be used, ignored or switched with another depending on needs.
Pro Resource efficient and fast
Due to its modular design, Postfix can be configured to only use parts that are required in each use-case and as such conserves resources and operates fast.
Pro Security focused
Postfix was built ground up with security in mind. It is highly secure. As such it can at times be a bit less flexible than Exim.
Pro Good documentation
Postifx has a thorough documentation with examples.
Pro Can run sendmail milters
Almost all Sendmail milters can be used with Postfix out of the box.
Pro Active development
New releases are typically 2-3 months apart.
Pro Sendmail compatible
Postfix is a drop-in replacement for Sendmail. It implements a compatibility interface with some options silently ignored.
Con No official version-controlled repository
See discussion at https://groups.google.com/forum/#!topic/mailing.postfix.users/6Kkel3J_nv4 and unofficial repo at https://github.com/vdukhovni/postfix
Con Pain in the neck to configure
Postfix is hard to configure: be prepared to spend some time configuring TLS and permissions, through trial and error. Postfix is an awesome MTA but if you are looking for a quick and functional setup, don't use it.