code: func NewSingleEmail(from *Email, subject string, to *Email, plainTextContent string, htmlContent string) *SGMailV3 {
  	plainText := NewContent("text/plain", plainTextContent)
  	html := NewContent("text/html", htmlContent)
  	return NewV3MailInit(from, subject, to, plainText, html)
  }
docs: // NewSingleEmail ...