claws is actually in the extras-devel repo for the n900 --- unfortunately it doesn't seem very stable - I set it up for my work account, and it seemed ok initially, but after 2 or 3 uses it now crashes everytime I open my mailbox. Unfortunately I haven't had time to try to debug it or fix it...

I also must admit that I don't find modest very usable - I believe there have been at least bug fixes to it in PR 1.2, but I doubt there have been major changes.