Test if your SMTP-server is modern
Can your mail server send to jörg at übungsgelän.de ?
gmail can. It means, it can resolve DNS via IPv6, deliver via IPv6 MX,
and handle Unicode in both domain-names and user-names.
if your mailer cannot, then try
- joerg at 4.übungsgelän.de
- joerg at xn--bungsgeln-22a9w.de
- joerg at 4.xn--bungsgeln-22a9w.de
- joerg at only6.zq1.de
- jörg at zq1.de
Why does this matter?
Since IPv4 addresses are depleted, not everyone can host his DNS or mail-server on a v4 address, but IPv6 addresses are plenty and available.
UTF-8 is the standard to represent any relevant characters on earth (even Klingon). Mail servers should not force people into an ASCII-only world.