ViewController

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
}

@IBAction func sendEmail(_ sender: Any) {

    sendEmail()

    }

func sendEmail() {
    if MFMailComposeViewController.canSendMail() {
        let mail = MFMailComposeViewController()
        mail.mailComposeDelegate = self
        mail.setToRecipients(["xxx@xx.com"])
        mail.setSubject("test  ")
        mail.setPreferredSendingEmailAddress("xxx@jxxx.com")

        present(mail, animated: true)  

    } else {
        //show failure alert
    }

    func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error?) {

        controller.dismiss(animated: true, completion: nil)

    }

}