Sylpheed, once mature, is the best. It has a small memory footprint, decent speed and works with SSL, IMAP-Folders, attachments etc. It has some bugs: You may not minimize it (wont show up in the tasklist), has some menues not hildonized and is not yet ready for Maeomo 2.0 / OS 2006.

I really need this app, so unfortunately I have to wait with the upgrade untill it is available for the new firmware.