I am the SECDIR reviewer for this draft.

I have a question, probably because I am not very familiar with the IMAP protocol. All my questions could be answered by stating "we're documenting existing practice" and I am fine with that: Why three bits for `MailFlagN` instead of a single digit?

The description of the keywords was clear and easy to understand, thank you.

Does the ordering of the keywords in Section 4 match some other IMAP documents? I would find it easier to read all the simple definitions together, and then the related words (memo, attachment, subscription).

Sec 4.9: "verified with complete confidence". Please strike the word complete.  In fact, as a security person, I strongly suggest removing almost all absolutism from this section: "absolute certainty," "strong signal," etc.

Sec 7 should probably mention that use and interpretation of these keywords, depends on the client/user ability to trust the IMAP server, and/or also refer to the security considerations in RFC 9051.