I embarked on setting up my own email server a few years back. I settled upon Courier as the heart of this configuration and never once regretted it. I came to rely upon it for all my daily email needs, including wiresless email via my Treo. Even friends and family started to get accounts on my system, since I was able to offer it cheaply, had plenty of space, and could easily adjust it for different needs. After a while, though, I did become increasingly concerned with running this setup from my home and the regular upkeep of things like spam filters, virus scanners, etc got to be a bit of a drag.
The answer was certainly to find a way to host the setup. The real question was which was the best manner in which to do so. I quickly ruled out dedicated hosting services due to cost. But would generic web hosting provide the flexibility and features I needed? Were there other options?
I searched first for providers that were recommended by others I knew and those that were known to be based upon the products I'd already trusted for so long. I set up trial service with web hosters, VPS (Virtual Private Servers, an inbetween generic hosting and dedicated servers), and two different email-only providers.
In the end, I decided that most generic hosting providers simply didn't offer as much as I wanted from my email services -- nothing even close to what I'd been running for myself. Virtual Private Servers, which permit "root" access to install whatever software you like, seemed like they could do what I wanted but they had the disadvantage of costing more and not providing nearly the resources I'd been used to in my own setup.
Finally, I decided that if I were going to host email that I would have to go with a vendor that did absolutely nothing except email and did it well. After trying a few I chose to go with Webmail.us. They are an email-only hosting company and they have a very solid reputation. In the year I have had my email hosted there, it has been down only once (and even then only for a few hours). They have incredibly good service, support an extremely wide variety of clients, and have a very respectable collaboration toolset that they develop and enhance in-house. In fact, there are times that I wonder if I should just switch to using their web-based client... but I'm still tied to my old favorites like Thunderbird for now.