I have also same problems as eg. lotech.
Mail client won´t update accounts as scheduled even when connected to network (3G) or it updates accounts and gives new mail announcement something like ones in a month
Manual update is also very slow process usually and when it is done mail client tells theres no new mails wich isn´t true because right after this when I go to check mails on webmail or PC´s client these show several mails that are arrived hours before. Only solution that I have found is disconnet from network and reconnect and voila now N900 mail client will find all those mail.