It seems you are confusing several things:
1.
Generally email headers should contain only ASCII data, thus when trying to store non-ascii data/text, you need to use some 'tricks', like encoded words:
=?utf-8?Q?V=E5rd?=
2.
iso-8859-1 is an 8-bit single-byte encoding intended for Western European languages.
3.
UTF-8 is a character encoding capable of encoding all possible characters, or code points, defined by Unicode.
4.
Please note that custom headers ('X-') don't support encoded words by default, so no client is going to parse and decode them automatically.
5.
Use HeaderEncoder class:
string encoded = new HeaderEncoder.Encode("hello åäö");