MailComposeViewController.swift

import UIKit
import MessageUI

class MailComposeViewController: MFMailComposeViewController, MFMailComposeViewControllerDelegate {
    func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error?) {
        controller.dismiss(animated: true) {
            switch result {
            case .cancelled:
                print ("Canceled")
            case .saved:
                print ("Saved")
            case .sent:
                print ("Sent")
            case .failed:
                print ("Error:", error ?? "nil")
            }
        }
    }
    convenience init?(to: [String], subject: String, body: String) {
        guard MFMailComposeViewController.canSendMail() else { return nil }
        self.init()
        mailComposeDelegate = self
        setToRecipients(to)
        setSubject(subject)
        setMessageBody(body, isHTML: true)
    }
}

to call it from your IBAction

@IBAction func ok(_ sender: UIButton) {

    if let mailComposer = MailComposeViewController(to: ["[email protected]"], subject: "Testing", body: "Testy test testing") {
        present(mailComposer, animated: true, completion: nil)
    }

}