Comment by cryptonector

No, RFC 3501 says

> The unique identifier of a message MUST NOT change during the > session, and SHOULD NOT change between sessions. Any change of > unique identifiers between sessions MUST be detectable using the > UIDVALIDITY mechanism discussed below. Persistent unique identifiers > are required for a client to resynchronize its state from a previous > session with the server (e.g., disconnected or offline access > clients); this is discussed further in [[IMAP-DISC](https://www.rfc-editor.org/rfc/rfc3501#ref-IMAP-DISC)].

so, "SHOULD NOT", but in practice it's really hard to make {UID, UIDVALIDITY} assignments persistent and unique, so IMAP servers don't, and as you can see, they are allowed to not.

I.e., it's perfectly compliant to generate a new UIDVALIDITY for each session and then assign UIDs to emails in folders when you open them