FYI, it would have been most helpful if you had posted your example in the issue text instead of as an attached file, as it explains the problem better than your text does :)

Here is a minimal reproducer:

>>> m = EmailMessage(policy=strict)
>>> m['From'] = '"Foo Bar, España" <foo@example.com>'
>>> bytes(m)
b'From: Foo Bar, =?utf-8?q?Espa=C3=B1a?= <foo@example.com>\n\n'

This serialization of the header is, as you say, invalid.  Either the comma should be encoded, or the "Foo Bar," should be in quotes.