When used in SASL, all commands and responses are BASE64 encoded
so they cannot violate SMTP syntax.
As for SMTP itself, the protocol is limited to 7-bit ASCII unless
client and server negotiate otherwise. If there is an effort to
extend SMTP with non-ASCII text in commands (as opposed to non-ASCII
text in message content) then I would appreciate a pointer.
(I am not interested in efforts to simply replace ASCII by UTF-8,
because that would result in non-interoperable implementations).