offlineimap smart backup synchronization = bidirectional synchronization without deleting any messages

[Edward Z. Yang]

Excerpts from proxym's message of Tue Dec 18 22:57:24 +0800 2012:
> After I changed `no-delete-local` back to `no` ALL previously
> not-deleted messages (because of `no-delete-local yes`) are deleted
> from my local repository.
> This patch IS TOO DANGEROUS in case of no back synchronisation of
> messages to remote imap repository.

Yes, what do you expect? OfflineIMAP sees that it's missing from the
remote and not from the local, and syncs accordingly.  The same applies
with maxage; if you unset it expect a lot of activity.

One way to prevent OfflineIMAP from deleting anything is to remove
your status folder; then OfflineIMAP assumes everything is "unsynced"
and won't remove anything.

Edward