从 swift 3 发送邮箱

我正在try 设置一个带有发送邮箱选项的应用程序.

我有这个密码:

import Foundation
import MessageUI
import UIKit

class emailClass: UIViewController, MFMailComposeViewControllerDelegate {
    override func viewDidLoad() {
        super.viewDidLoad()

        if !MFMailComposeViewController.canSendMail() {
            print("Mail services are not available")
            return
        }        
        sendEmail() 
    }

    func sendEmail() {      
        let composeVC = MFMailComposeViewController()
        composeVC.mailComposeDelegate = self
        // Configure the fields of the interface.
        composeVC.setToRecipients(["address@example.com"])
        composeVC.setSubject("Hello!")
        composeVC.setMessageBody("Hello this is my message body!", isHTML: false)
        // Present the view controller modally.
        self.present(composeVC, animated: true, completion: nil)
    }

    func mailComposeController(controller: MFMailComposeViewController,
                           didFinishWithResult result: MFMailComposeResult, error: NSError?) {
        // Check the result or perform other tasks.
        // Dismiss the mail compose view controller.
        controller.dismiss(animated: true, completion: nil)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

所以我收到了这样一条信息:"邮件服务不可用".