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