Looks like a nice tool for syncing mailboxes locally.

I don't think that's the only challenges but those are indeed two good things to keep in mind to ease the syncing.

This checksum approach has interesting properties. Can come in handy when syncing.

Looks like a nice tool for real-time collaboration on files.

A follow up on the Dropbox sync engine rewrite, or why they introduced brotli compression.

Good insight into why Dropbox rewrote its sync engine for desktop clients.

Testing sync engines can be especially challenging. This story from Dropbox gives ideas on how to do it well.

If you wonder how the protocol is designed and how the actual implementation works, this is a nice introduction. Clearly it's helped by the size of that program which is fairly small.

Interesting dimensions to use when classifying syncing solutions and to see which ones will meet your constraints.