mail/internal/mail/message.go
func (m *Message) SetBody(contentType, body string, settings ...PartSetting) {
	m.SetBodyWriter(contentType, newCopier(body), settings...)
}
func (m *Message) SetBody(contentType, body string, settings ...PartSetting) {
	m.SetBodyWriter(contentType, newCopier(body), settings...)
}
// SetBody sets the body of the message. It replaces any content previously set
// by SetBody, SetBodyWriter, AddAlternative or AddAlternativeWriter.