| Subject: | To header should not contain an encoded value |
perl -MCourriel::Builder -E 'say build_email( to( q{sástien.lambert@example.com} ), subject( q{Testing} ), from( q{autarch@urth.org} ), plain_body(q{Body}) )->as_string'
produces
To: =?UTF-8?B?c8ODwqFzdGllbi5sYW1iZXJ0QGV4YW1wbGUuY29t?=
This is not valid (and probably applies to other parts of the headers as well).