Special characters in email headers are normally put within double quotes. However, encoded words (=?charset?x?...?=) are not allowed withing double quotes. When the header contains a word with special characters and another word that must be encoded, the first one must also be encoded.

In the next example, The From header is quoted and therefore the comma is allowed; in the To header, the comma is not within quotes and not encoded, which is not allowed and rejected.

From: "Foo Bar, France" <foo@example.com>
To: Foo Bar, =?utf-8?q?Espa=C3=B1a?= <foo@example.com>