Email messages crash on non-ASCII domain when email encoding is non-unicode. Description

The task

When the computer hostname is set in unicode (in my case "正宗"), the following test fails: ​https://github.com/django/django/blob/master/tests/mail/tests.py#L368 Specifically, since the encoding is set to iso-8859-1, Python attempts to convert all of the headers to that encoding, including the Message-ID header which…