so, here's my issues with hosting your own mail. it's probably the most difficult common internet service in the world to maintain. there are a litany of sysadmin tasks you must take care of before you even begin with the really tricky problems. the main reason is that e-mail is arguably the most critical of services. you can't afford to miss a critical e-mail, so its imperative it be incredibly robust and fault tolerant.

first you have to set up redundant hosts. preferably on different backbones in different colos, unless a majority of your gear is in one place. next you set up the software as secure as you can so there is a minimum window for 0-days and patching can be done in scheduled maintenance windows. then you set up your monitoring to notify you when something goes south, on both hosts. things like queue size checkers and threshold-based statistic alerts are handy.

next you set up your spam filtering. THIS IS HARD. i don't care what any old salty dog mail admin tells you, it is an arduous process to properly manage the filtering and blocking and unblocking etc of mail. for a small site, getting aol or yahoo to unblock your ip range may be a great feat. for a large site, 90% of your traffic will be spam and getting every legitimate mail is nearly impossible without subjecting your users to tons of crap.

the cost in money and time just to host some free webmail yourself is astronomical in comparison to putting up with an ad and a possible privacy violation from a free provider like yahoo or gmail. if you want your mail handled properly, pay a big e-mail provider to host it for you, or spend a lot moree time and money to do the same thing yourself.