[brongondwana]

That's not strictly true. IMAP has two things, UIDs which are mostly static (there's a UIDVALIDITY cache invalidation key to let a client know that UID information has been lost and recalculated); and message numbers.

UIDs don't change, but of course they can be deleted so it's a gappy list, meaning you can request even quite a large looking range of UIDs and get nothing back.

Message numbers change in every session, and also change every time you get an EXPUNGE. They're basically an ordered list without gaps, so you do a memmove at the offset of the EXPUNGE each time you get an expunge.

There are efforts like UIDONLY (RFC9586) to avoid having to keep that mapping at all, and there's OBJECTID (RFC8474) to let you cache a lot more even when UIDs are changed or when messages are moved between folders.