Messages without a Message-ID are ignored
Since the Message-ID header is the unique key used to synchronize folders, if a message on the source does not have it, the message will be silently ignored.
We would need a way to generate a "replacement" message-id with the following characteristics:
- unique to the message within the folder
- reasonably cheap to generate (it'll have to be computed every time we sync the folder)
- stable across invocations (or messages will be downloaded multiple times).