we already supported special-use flags. settable through the webmail interface,
and new accounts already got standard mailboxes with special-use flags
predefined. but now the IMAP "CREATE" command implements creating mailboxes
with special-use flags.
to get the security benefits (detecting mitm attempts), explicitly configure
clients to use a scram plus variant, e.g. scram-sha-256-plus. unfortunately,
not many clients support it yet.
imapserver scram plus support seems to work with the latest imtest (imap test
client) from cyrus-sasl. no success yet with mutt (with gsasl) though.
the idea is that clients may not support SCRAM-SHA-256, but may support
SCRAM-SHA-1. if they do support the 256 variant, they'll use it.
unfortunately, thunderbird does not support scram-sha-1 either.
