Another vote for Claws (I've converted to it 10 minutes ago, and I love it).

Modest wasn't handling very well my huge IMAP mailboxes (at least 5'000 mails in the Inboxes, more than 10'000 in a few other sub-folders).

The only issue I have with Claws is that the interface feels cramped on the N800. It's really a desktop application ported to the N800 without taking too much into consideration the form factor of the ITT. But I won't whine for that: all the other features are really excellent!