Browsing through mailcore2 [1], it looks like this would not support arbitrary IMAP flags. So far I have found only a handful of mail clients that can support

Browsing through mailcore2 [1], it looks like this would not support arbitrary IMAP flags. So far I have found only a handful of mail clients that can support them: MailMate, MailTags (brittle extension for Mail.app), and support for flags through folders if you happen to use Gmail as a server. 1: https://github.com/MailCore/mailcore2/blob/f708ce74e23b61ec6e5ae958eba0b8bcd8831a1e/src/java/com/libmailcore/MessageFlag.java#L5-L19 https://github.com/MailCore/mailcore2/blob/f708ce74e23b61ec6... EDIT: added reference.