Mailsync is a way of synchronizing a collection of mailboxes. The algorithm is
a 3-way diff. Two mailboxes are simultaneously compared to a record of the
state of both mailboxes at last sync. New messages and message deletions are
propagated between the two mailboxes. If you're familiar with CVS, it's the
same principle, except there's no opportunity for conflicts.

Homepage:
https://mailsync.sourceforge.net/
