The UIDVALIDITY is actually a great design. IMAP servers will usually store the UID mapping in some kind of index that is used even across restarts. Therefore i

The UIDVALIDITY is actually a great design. IMAP servers will usually store the UID mapping in some kind of index that is used even across restarts. Therefore in normal operation this is not a problem. When did you encounter problems with this in every day use? If the UIDs were not allowed to change, you would run into practical problems on edge cases. What if you switched your IMAP server implementation? What if the index files on server side got corrupt and you had to rebuild them? What if you had a fatal disk failure and had to restore an older backup? Mismatching UIDs would be even worse than a way to indicate to the client that the previously retrieved UIDs and associated messages are no longer valid. UIDVALIDITY is basically just a way for cache invalidation.