mail/internal/mail/message.go
func (m *Message) Reset() {
	for k := range m.header {
		delete(m.header, k)
	}
	m.parts = nil
	m.attachments = nil
	m.embedded = nil
}
func (m *Message) Reset() {
	for k := range m.header {
		delete(m.header, k)
	}
	m.parts = nil
	m.attachments = nil
	m.embedded = nil
}
// Reset resets the message so it can be reused. The message keeps its previous
// settings so it is in the same state that after a call to NewMessage.