You can also run alpine, the one and only usable mail client.

It's somewhat strange and not always user-friendly, but it's the only mail client (on any platform) that properly supports IMAP and doesn't break when handling tens of thousands of messages.

Sadly, it's not packaged properly for the N900, you need to download a precompiled .tar.gz binary...

http://home.mminternet.com/delaroca/index.html/