MailComposable.swift

import UIKit
import MessageUI

struct MailInfo {
    var recipient: String
    var subject: String?
    var message: String?
    var isHTML: Bool = false
}

protocol MailComposable: UIViewController {
    var mailInfo: MailInfo { get }
}

extension MailComposable where Self: UIViewController {
    func openMailApp() {
        let email = "mailto:\(mailInfo.recipient)"
        guard let url = URL(string: email) else { return }
        if UIApplication.shared.canOpenURL(url) {
            UIApplication.shared.open(url, options: [:], completionHandler: nil)
        }
    }
}

extension MailComposable where Self: UIViewController & MFMailComposeViewControllerDelegate & UINavigationControllerDelegate {
    func compose() {
        guard MFMailComposeViewController.canSendMail() else { openMailApp(); return }
        let mailComposorVC = MFMailComposeViewController()
        mailComposorVC.setToRecipients([mailInfo.recipient])
        if let subject = mailInfo.subject {
            mailComposorVC.setSubject(subject)
        }
        if let message = mailInfo.message {
            mailComposorVC.setMessageBody(message, isHTML: mailInfo.isHTML)
        }
        mailComposorVC.delegate = self
        self.present(mailComposorVC, animated: true, completion: nil)
    }
}

Usage

class ContactUsViewController: UIViewController, MailComposable {
    var mailInfo: MailInfo {
           MailInfo(recipient: "[email protected]")
    }

    func contactsUsAction() {
          self.compose()
    }
}

extension ContactUsViewController: MFMailComposeViewControllerDelegate, UINavigationControllerDelegate {
    func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error? {
        //.. all the action you need to perform after.
    }
}