When I found preinstalled on my server Microsoft Exchange, I did not search for a better one, maybe because I'm lazy, maybe because it's the most trusted and used mail server in the world; for example Microsoft is using a fork of that for manage its hotmail, with 1.3 billions mail accounts, 3 billions of daily mail, 155 petabyte of storage (that's 155 million gigabytes!); and even Facebook uses Exchange (link in Italian - I could not find an English link) for its internal mail, even if the Facebook guys are for linux/opensource solutions.
So, I chose to use Exchange. And then I immediately regret my decision: the configuration it's a mess, and the user manual that it's a free download from Microsoft it's over 500 pages, and the default settings are less than ideal (It's so cool to be an open relay for spam, right?)
And what about options and functionality? It offers less than basic functions, unless you pay $$$ for third-party plugins. And its webmail works correctly only with Internet Explorer 6, etc...
So, yesterday I tried to disable it and replace with the most used opensource mail server:
Wow! Less memory usage with more functionality!
Bye bye Exchange, I won't miss you!
Update: wow, after a few reboots and the complete uninstall, the memory usage has reduced in half! 
