Why krakozyabry in the subject line in "Opera Mail"?

Question

go

I wash the letter:

var msg = []byte("To: " + myMail + "\r\n" +
  "Subject: " + "Тема письма" + "\r\n" +
  "Это содержимое письма.")
var err = smtp.SendMail(HostPortSMTP, auth, MailFrom, []string{myMail}, msg)

As I understand it, it leaves in UTF-8 encoding ... And on the web page - on the mail server - everything looks fine, but in the mail client "Opera Mail" in the Subject [instead of Cyrillic] krakozyabry.
How to decide? How is it generally done? Maybe the encoding required for the client is somehow requested?
After all, incoming letters in this client are displayed normally.

Answer 1

Headers with non-ASCII characters must be encoded according to the standard ( 1 , 2 ). I doubt go will do it for you.