Alternatives to Platemail
There are of course many other (and currently, much better) pieces of email software available. This page identifies some of the bigger players, and details why I don't use them instead of writing my own. Note that I don't expect everyone to agree with these opinions, I'm simply justifying my own choice.
Microsoft Outlook: Many people these days don't need a reason to avoid Microsoft products. Microsoft have something of a reputation for releasing extremely buggy software, or software with gaping security flaws. The truth of this is no longer of any interest to me personally, as some time ago I switched to using Linux on my desktop. As Outlook isn't available for Linux, it isn't really a contender.
Being a commercial product, you also have to pay for Outlook. Microsoft also offers Outlook Express, a free, slightly cut down version of Outlook. The last time I used Outlook Express properly, it seemed to me to have serious issues with IMAP support, mail filtering, and the handling of attachments. While it may well have moved on in these areas since then, it remains the case that many of the viruses doing the rounds are written to exploit vulnerabilities in Outlook, and as such it probably wouldn't be my first choice, even under Windows.
KMail: This is actually the odd-one out of the bunch, because I actually do use this client. However, it's only until Platemail reaches the stage where it is usable; at which point I will abandon KMail.
KMail is generally a decent email client for Linux, although there are certain things that it doesn't currently do well; in particular the IMAP support is a little half-hearted, so for example you can't automtically filter messages out of an IMAP mailbox.
One of the other slight greivances I have with KMail is its dependence KDE - KDE is a pretty massive package, and at time of writing you can't download KMail as a stand-alone application. While it is understandable that KMail requires packages such as Qt to run, a greater level of separation must be possible. KMail is also restricted to Linux/UNIX platforms, which means that it isn't an option for Windows users.
Alternatives I haven't looked at yet
Pegasus, Foxmail, Calypso