Unity connection 10.x does not like TLDs with more than 3 characters. However, longer TLDs have been coming. E.g. in Switzerland, you can now register .swiss as a TLD. Consequently, an email that ends in .swiss must be supported, yet it isn't:
URL Sent: https://chdevcucn10.nxodev.intra:8443/vmrest/users/ff210a3e-c9dc-41f6-afa5-4092fed7a8e7/messagehandlers/42589c68-51ba-41ca-b4fb-5067e102433b
Method Sent: PUT
Body Sent: <MessageHandler><VoicemailAction>3</VoicemailAction><RelayAddress>stephan.steiner@nxodev.swiss</RelayAddress</MessageHandler>
Success returned: False
Status returned 400:BadRequest
Error Text: Bad Request
Raw Response Text: <?xml version="1.0" encoding="UTF-8" standalone="yes"?><ErrorDetails><errors><code>DATA_EXCEPTION</code><message>Enter Vaild Relay Email Address</message></errors></ErrorDetails>
Total object count: 0
Status description: Bad Request
So no more "RFC so and so says 3 letter TLDs".. if you can register longer TLDs, they must be supported (and you can enter those adreses just fine using the GUI).