change of UIDVALIDITY shouldn't be considered fatal for the imap connection.
maybe the error handling needs to be cleaned up in general.

don't require maildir_trash. currently MaxMessages gets into the way of
simply removing it; that is fixable with some shuffling, though.

refactor mailbox support. create proper mailbox drivers; handle imap and
maildir (and anything else) symmetrically; decouple UID->message mapping
from sync database - should use the same UID storing schemes as c-client
(pine, uw-imap) does, at least optionally, i think.

add asynchrounous operation to remote mailbox drivers. this is actually
what prevents us from simply using c-client for the previous point and
thus simply becoming mailsync.

store message flags in sync database, so _un_setting them will be properly
synced as well.

handle custom imap flags. currently, isync just fails horribly if it
encounters some.

add option to suppress upload of messages (but not deletion).

add support for syncing with other: and shared: via NAMESPACE

isync ignores asynchronous notifications (untagged responses), so mail 
arriving during a fetch will not be fetched in the current run any more.

add a way to automatically create and sync IMAP subfolders.

possibly timestamp mails with remote arrival date.

possibly recover from UIDVALIDITY change by resyncing according to message
IDs - this is a pretty common condition with uw-imap.

possibly use ^[[1m to highlight error messages.