Comment by actuallyalys
3 years ago
There's arguably a third: Message-Id. It's meant to be globally unique and comes from the message itself. I don't know how workable it is for email synchronization with IMAP; IMAP servers will provide them (they're just another header) but I'm not sure the client commands exist for a message-id approach to be as efficient or ergonomic.
Message-IDs aren't workable as unique IDs in an email client. There are a few ways where two different messages can end up with the same message-id (e.g., multiple drafts, or the copy in the sent folder versus the copy in your inbox), where the differences are not really relevant for the purposes message ids are used for (e.g., threading).