> The notmuch tag database is not a standard IMAP concept This is not quite true. IMAP has a notion of “keywords”, which are arbitrary strings, that you can as

> The notmuch tag database is not a standard IMAP concept This is not quite true. IMAP has a notion of “keywords”, which are arbitrary strings, that you can associate with any message. This is described in the IMAP RFC [0]. Unfortunately, very few email web applications expose this feature in their UIs so it remains obscure. FWIW Google Mail does store unlimited keywords on the server. So does Fastmail but last time checked (years ago) it only supported a maximum of 128 per IMAP mailbox. 0: https://www.rfc-editor.org/rfc/rfc3501#section-2.3.2 https://www.rfc-editor.org/rfc/rfc3501#section-2.3.2