With Mailgun, it is possible to utilize IDN-based headers in an email message. However, it is not possible to utilize IDN-based domains within the Mailgun environment.
For full details on Internationalization and Mailgun, please visit our User Guide.
Understanding The Context
Setting The Scene
Mailgun does support the use of Internationalized Domain Names (IDNs) in the messages themselves. For instance, internationalized email addresses in the To and From fields are fully supported in the Mailgun environment. The same is true of subjects that contain UTF-8 characters.
Mailgun, however, does not support domains that consist of Punycode as well as Cyrillic and other non-ASCII alphabets.
Discerning The Nuance
There still are relatively few mailbox providers that fully support UTF8SMTP, without which IDNs cannot be supported. IDNs are a feature that requires both parties - the sending and receiving providers - to operate according to a common standard or protocol: UTF8SMTP, in this case. Until the time arrives that IDNs are more widespread, you may find the use of such domains problematic.
Also of note is that while the Control Panel and Domains API may technically allow non-ASCII domains to be created, numerous Mailgun features such as Mailing Lists, Routes, and SMTP Credentials will not function correctly with non-ASCII domains.
Implementing The Solution
To ensure a domain and all of its associated features operate as expected, please create a domain that utilizes only ASCII alphanumeric characters (a helpful chart of ASCII can be found here).
Sinch Mailgun has answers! If you have any concerns or questions, please send us a Support ticket using the Support page within your Mailgun Control Panel. Our Support Team will be happy to assist!