Uberspace mail

If there's no MX pointing to the Uberspace server, every mailbox will end up on @$USER.uber.space

adresses like .$USER.uber.space (i.e. created as punycode variant xn--du8h are inconsistent between server and client. (src)
Here I just have to experiment more.
Because of the parallels to domains I assume that punycode can also be applied to mail addresses and that emojis can be used as normal ASCII chars.