when seeking for syncing two imap maildir folders one has the choice among many programs such as imapsync, isync, offlineimap ... But all of these provide individually only a few options.
For example syncing a folder by only appending unexisting files, i.e. without deleting anything seem to be supported by no one real imap syncing software stable release. Here "real syncing" means a process that first collects the data it needs about the files in the destination folder and proceeds with syncing with the requested options. For example isync does indeed provide an option for only appending unexisting files in the destination folder but despite its name it does not seem to care about the content of the destination folder. The first time it runs it copies all the emails from the source folder and writes in a text file what it did to avoid duplicates for the next times it will run. For syncing two folders that initially contain common stuff this would eventually result in duplicate emails in the destination folder. Moreover each of these programs have their own configuration style that takes a lot of time to learn. Regarding the many features rsync provides for syncing files and its ease of use, adding to it the ability to sync imap maildir folders will probably make of it a definitive choice for this purpose.
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