The two major ones, UWash and Cyrus, don't play on Windows and require you to have a separate Mail Transfer Agent (sendmail) installed and configured. Well, I'm not up to the task of setting up an MTA on my Linux server (it's pretty amazing that I even have a Linux server), and I imagine there are many other people out there like me.
So I'm writing an IMAP server in Java that hopefully will be so easy to install and use, anyone can set it up and run it off their home machine. (The DynDNS part of it will actually be the harder part to get newbies through.) I'm calling it the "Poorman's IMAP Server" because it's not targeted toward large, production environments; it should be high performance but I'm not going to spend a lot of time worrying about >20 user environments. That energy will instead be diverted to making it correct, configurable, flexible, and easy to use.
The default mode will be "piggybacking" off a POP account, so rather than requiring an MTA to be installed you'll need to have a POP account somewhere. For users who want to set up a standalone server, this may sound like a backward approach compared with integrating with an MTA/mailstore, but I'm guessing this approach will in fact be much easier for many users. This is especially true on Windows where there don't seem to be any mailstore standards, but a plethora of free SMTP/POP implementations.
Another thing that is nice about piggybacking off POP is that the machine that hosts the IMAP server doesn't necessarily have to have stupdendous uptimes. If the only "server" you have access to is your home machine, which is connected via a sometimes-flaky DSL line, then you don't have to worry about e-mails getting bounced back to their senders because your DSL is down. Instead, your ISP's POP machine, which should have much better availability, will store the message for as long as it takes for your IMAP server to get back online. As an added bonus, you don't have to change the e-mail address you've been using.
I realize you can use fetchmail or something similar and enjoy the same benefits. But then, that is one more piece of Windows-unfriendly software to install/configure.
Feedback is welcome. Thanks...