Thunderbird still hasn't implemented proper support for internationalized domain names (IDNs) as these two bugs show: Bug 235312 from 2004, Bug 923043 from 2013.
In brief, sending messages to mail addresses at internationalized domains may fail, depending on whether the final server supports so-called SMTPUTF8 (RFC 6531). If it doesn't, it likely also doesn't support UTF-8 headers (RFC 6532), and delivery will fail due to raw UTF-8 in addressing headers. This especially applies to domains hosted at IONOS.
Mail clients like Microsoft Outlook and FairEmail handle the situation somewhat better, since they unconditionally force internationalized addressing headers to ASCII-Compatible Encoding (ACE), which uses Punycode as encoding.
Betterbird has now introduced the preference mail.strictly_ace (similar to mail.strictly_mime which avoids 8bit characters in mail bodies). With this preference set, Betterbird will behave like the two mentioned mail clients and force addressing headers to ACE.
Further reading: