Swift 3 email form cannot send out - ios

Question

i am doing a form using textfield and textview
after i key in all the information and click the submit button, the app will close and jump to my viewcontroller.swift and show like the below photo
and this is my viewcontroller.swift
import UIKit
import MessageUI
class ContactViewController:UIViewController,MFMailComposeViewControllerDelegate,UITextViewDelegate,UITextFieldDelegate {
#IBOutlet weak var message: UITextView!
#IBOutlet weak var contactSubject: UITextField!
#IBOutlet weak var contactNumber: UITextField!
#IBOutlet weak var contactEmail: UITextField!
#IBOutlet weak var contactName: UITextField!
#IBOutlet weak var contactDes: UILabel!
#IBOutlet weak var contactTitle: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
contactTitle.text = "SEND US A MESSAGES"
contactTitle.sizeToFit()
contactDes.text = "Do you have anything in your mind to tell us?\nPlease don’t hesitate to get in touch to us via our contact form."
contactDes.sizeToFit()
//Contact Form
contactName.delegate = self
contactEmail.delegate = self
contactNumber.delegate = self
contactSubject.delegate = self
message.delegate = self
self.message.isUserInteractionEnabled = true
self.message.isEditable = true
let borderColor : UIColor = UIColor.init(red: 46.0/255.0, green: 49.0/255.0, blue: 146.0/255.0, alpha: 1)
contactName.layer.borderWidth = 1
contactName.layer.borderColor = borderColor.cgColor
contactEmail.layer.borderWidth = 1
contactEmail.layer.borderColor = borderColor.cgColor
contactSubject.layer.borderWidth = 1
contactSubject.layer.borderColor = borderColor.cgColor
contactNumber.layer.borderWidth = 1
contactNumber.layer.borderColor = borderColor.cgColor
message.layer.borderWidth = 1
message.layer.borderColor = borderColor.cgColor
}
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
contactName.resignFirstResponder()
contactEmail.resignFirstResponder()
contactNumber.resignFirstResponder()
contactSubject.resignFirstResponder()
return true
}
#IBAction func submitbtn(_ sender: Any) {
let mailVC = MFMailComposeViewController()
mailVC.mailComposeDelegate = self
mailVC.setSubject(contactSubject.text!)
let email = contactEmail.text!.lowercased()
let finalemail = email.trimmingCharacters(in: NSCharacterSet.whitespacesAndNewlines)
let mailcontent = "Name :\(contactName.text!)\n\n Email:\(finalemail)\n\n Contact Numer :\(contactNumber.text!)\n\n Subject :\(contactSubject.text!)\n\n Message :\(message.text!)"
mailVC.setMessageBody(mailcontent, isHTML: false)
let toRecipent = "abc#example.com"
mailVC.setToRecipients([toRecipent])
self.present(mailVC, animated: true, completion: nil)
}
}