IMAP Keywords are part of RFC-3501 and although RFC-3501 considers them optional the RFC-5788  describes them as a sudo standard for IMAP server implementations. Smarter Mail does not implement this part of the RFC. Given that the mechanism to keep system flags on the server is not optional there is already a mechanism available to keep IMAP keywords so implementation should be trivial. As this is the industry standard of an IMAP server the omission of keywords should be considered a bug in the implementation.