I've been using the N900 with a IMAP server (dovecot) since I got it. My inbox isn't huge though, and I have about 20 folders. I try and sort things as much as possible. Thus far the N900 has been perfect. I've been able to check email, reply, etc. And it roams off the WiFi onto the 2G network (AT&T) and I've yet to have the email stall or fail to pickup any new emails that have arrived. Granted everyone has had different experiences, but mine has been top notch. I don't see myself switching to anything else.