If you’ve been one of the people trying to use Gmail via IMAP on your Windows Mobile device (and experiencing quite a few weird issues), the Google Mail team have news for you — they’ve fixed the problems.
Link: Official Gmail Blog: IMAP on Windows Mobile devices
There are a lot of email clients out there, each with its own quirks, so making IMAP work across all of them can be tricky. We’ve worked with Microsoft engineers to debug the incompatibilities with Windows Mobile and made some tweaks to our code. As some people have already noticed, these fixes went out last week, so you should now have a better experience reading and sending Gmail messages using IMAP from your Windows Mobile device. If you haven’t yet set up IMAP, just follow the configuration instructions to get started.
The configuration instructions are here.