I use k-9 mail these days, which is OK but doesn't handle huge folders well. However, back when I had an n900 with its hardware keyboard, my mobile mail client was ssh, screen, and mutt.
not the commenter you want, but i had both a mobile client (if i am using mutt, that means i can access the imap/pop server from other clients) and the ultimate ssh client on my phone which gave me full mutt, in case i needed to have my procmail filters helping me)
Navigating any terminal software without a true keyboard is an exercise in frustration. Look, I understand what you're saying, but claiming that an ssh client on a phone is a full-fledged replacement for a touch-optimized UI is disingenuous.