I've set up a test address for the SMTPUTF8 extension created by the IETF EAI working group.
If you send mail to jøran@blåbærsyltetøy.gulbrandsen.priv.no Jøran will send you a stock reply, which you can use to test that unicode mail works in both directions.
For the moment you must be able to send via IPv6. Jøran can send the reply back via either IPv4 or IPv6, but you have to send the initial message via IPv6. I intend to add a v4-capable secondary MX later.
I have or can arrange other testing too; send me mail if you're interested.