mail/internal/mail/message.go
func (m *Message) SetHeader(field string, value ...string) {
	m.encodeHeader(value)
	m.header[field] = value
}
func (m *Message) SetHeader(field string, value ...string) {
	m.encodeHeader(value)
	m.header[field] = value
}
// SetHeader sets a value to the given header field.