Very nice Copernicus

Offline IMAP would allow you to sync mail clients on separate machines eg you move a message on modest to a local folder on the N900 it will automatically do this on your desktop or laptop as well. Complete info is here http://offlineimap.org/

Tree view allows quicker access to different folders of different email accounts. Rather than having to go in to an individual account to access say a local folder it is available on the first screen of modest. See below that is how modest starts with tree view.

edit btw updated the wiki with above info.