I have been using the 6810 mail client for 6 months over IMAP. It works , pretty reliably BUT its very basic. I have used a lot of IMAP4 mail clients including small PDA ones (Palm mostly). I think the mail client could be great and provide a true mobile communication device but needs some extra features
1) HTML email. I get a lot of this, about 1 in 3 messages where no TEXT rendition of the body exists, only HTML. This is a sender defined option so I cannot control it. The problem is the mail client does not do any HTML filtering. This means that many messages consist of just raw HTML. A basic HTML filter (ideally during download) would be good. I recall a short C program called 'less' which converted HTML to formatted text. That is what it needs.
2) True IMAP. The client supports IMAP but only offers POP3 functionality. For example I cannot see any server-side folders, not perform actions on server-side messages. If I delete mail on the Nokia, this action is not replicated back to the server. It's basically POP3 over via IMAP4 commands. Some really small IMAP clients have been built which offer full IMAP4 search and the ability to execute actions against the IMAP4 server (move, delete, download) check out PaPimail for Palm.
3) configurable POLL period. Many phones (Sony for example) can be configured to auto-poll for new mail at a configurable interval i.e every hour. This would provide a way of having a 'blackberry-like' experience where mail appears to show up without user intervention i.e. dialling etc.
4) better integration with other apps. I'd like to save a message in a calendar event or SMS message. This works well from SMS (save to calendar etc) but email feels like a bolt-on and doesn't really work well with the other tools