I've had all those problems that have been posted here. In my case the solution has been to use a Nokia Messaging account in order to integrate my mails accounts. I've setted those in my N900 and now it seems other!! Mail faster, posiblity of choosing hours for sync and few more features has been added to my device!!.

It may seems strange but I would say that all my phone performance has been improved, not only the mail cient!!

The unique problem that I've found with Nokia Messaging is that mail widget only show the number of unread messages and no longer shows the subjects in messages.