Flowmailer follows this RFC, but does not support SMTPUTF-8 special characters.
In practice, that means you can use special characters (ü, á, things like that) in headers, but their use is limited in the email address. For most of our customers, an SMTP UTF-8 character is a typo and opens people up to phishing. Most mail servers and forms do not accept those characters.
This means the local part (you@) does not support special characters, even when encoded as --.
The domain (flowmailer.com) supports international notation such as xn--vk-kchen-95a.de, but it does not support UTF-8 legible umlauts, accents, et cetera.