[bodyfour]

The reinjection on clash (CSMA/CD for numbers :-) works in close-knit clusters but has issues with network splits. Imagine one server is in SF and the other is in Amsterdam, and a network problem makes them unable to talk for a few hours. In the mean time, mail gets delivered and read on each server. Then they reconnect to each other and you find a storm of conflicts to resolve.

Also, I strongly believe that per-message UUIDs should always start with the delivery time. That way as long as your servers are all reasonably synchronized you can get reasonable "sort by folder order" behavior without any sequential ID.

You're right that the modseq issue is hard, but the Merkle tree suggestion from @jorangreef can address that. Refer to my reply to him for more details.