Flowmailer follows the RFC: https://datatracker.ietf.org/doc/html/rfc5322 , but we don't support SMTPUTF8.
In practice, that means you are allowed to use foreign characters in headers, but only limited in the mail address.
We do that because it's often a typo anyway, phishing is a real danger with addresses with an accent oid, and most mail servers and online forms don't accept them anyway.