How to add a bcc using golang standard `net/smtp`?

I am trying to add a list of bcc using go's net/smtp package.

    to := []string{"recpt@gmail.com"}
    msg := []byte("To: recpt@gmail.com\r\n" +
                  "cc: cc@gmail.com\r\n" +
                  "bcc: bcc@gmail.com\r\n" +
                  "Subject: TEST SUBJECT\r\n" +
                  "\r\n" +
                  "Test Email Body")
    err := smtp.SendMail("mailserver.com:25", auth, "sender@gmail.com", to, msg)

the bcc part doesn't work.

P.S. I don't want to use any external lib.