mail/internal/mail/message.go
func (m *Message) AddAlternative(contentType, body string, settings ...PartSetting) {
	m.AddAlternativeWriter(contentType, newCopier(body), settings...)
}
func (m *Message) AddAlternative(contentType, body string, settings ...PartSetting) {
	m.AddAlternativeWriter(contentType, newCopier(body), settings...)
}
// AddAlternative adds an alternative part to the message.
//
// It is commonly used to send HTML emails that default to the plain text
// version for backward compatibility. AddAlternative appends the new part to
// the end of the message. So the plain text part should be added before the
// HTML part. See http://en.wikipedia.org/wiki/MIME#Alternative