Actually, it seems to work using the punny code as the domain name in smartermail.
I was able to send a message to the IDN domain name I added.
However when I reply to the received message, from the webmail, the "from" address used is the punny code instead of the IDN accented e-mail address.
To be honest, this whole IDN domains thing is a pain and I wish it never existed. But still we have customers that want to use it :/
Sébastien Riccio
System & Network Admin
https://swisscenter.com