Any email provider would be short of useless without supporting flags from IMAP, as that's what email clients use to see if an email has been read or not, if I'm not mistaken.
AFAIK, Migadu supports most if not everything from IMAP. I'm using tags via Thunderbird and have no issues syncing it between multiple computers and Migadu.
AFAIK, Migadu supports most if not everything from IMAP. I'm using tags via Thunderbird and have no issues syncing it between multiple computers and Migadu.