mail/internal/mail/message.go
func (m *Message) AddAlternativeWriter(contentType string, f func(io.Writer) error, settings ...PartSetting) {
	m.parts = append(m.parts, m.newPart(contentType, f, settings))
}
func (m *Message) AddAlternativeWriter(contentType string, f func(io.Writer) error, settings ...PartSetting) {
	m.parts = append(m.parts, m.newPart(contentType, f, settings))
}
// AddAlternativeWriter adds an alternative part to the message. It can be
// useful with the text/template or html/template packages.