web-app-template/backend/app/app.SMTP.Service/SMTPClientImpl.cs

using System;
using System.Net;
using System.Threading.Tasks;
using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;
using MimeKit.Text;
using NLog;
using app.Configuration;
using app.Model.Filters;
using app.SMTP.Interface;
using app.SMTP.Model;

namespace app.SMTP.Service
{
    public class SMTPClientImpl : ISMTPClient
    {
        private readonly EMailSettings _emailSettings;
        private static Logger _logger = LogManager.GetCurrentClassLogger();

        public SMTPClientImpl(EMailSettings emailSettings)
        {
            this._emailSettings = emailSettings;
        }

        public async Task SendEmailAsync(EMail email)
        {
            var emailMessage = new MimeMessage();

            emailMessage.From.Add(new MailboxAddress(_emailSettings.FromName, _emailSettings.FromAddress));
            emailMessage.To.Add(new MailboxAddress("", email.Receiver));
            emailMessage.Subject = email.Subject;
            emailMessage.Body = new TextPart(TextFormat.Html) { Text = $"{email.Message}<br><br>Dies ist eine Automatische E-Mail.<br>Gesendet von Login counter @ {System.Net.Dns.GetHostName()}" };

            using (var client = new SmtpClient())
            {
                client.Timeout = 30000;
                try
                {
                    await client.ConnectAsync(_emailSettings.MailServerAddress, _emailSettings.MailServerPort, SecureSocketOptions.Auto).ConfigureAwait(false);
                }
                catch (Exception ex)
                {
                    _logger.Error(ex);
                    throw new ServiceException(ServiceErrorCode.ConnectionFailed, "Connection to mail server failed");
                }

                try
                {
                    await client.AuthenticateAsync(new NetworkCredential(_emailSettings.Username, _emailSettings.Credentials));
                }
                catch (Exception ex)
                {
                    _logger.Error(ex);
                    throw new ServiceException(ServiceErrorCode.InvalidUser, "Authentification to mail server failed");
                }

                try
                {
                    await client.SendAsync(emailMessage).ConfigureAwait(false);
                }
                catch (Exception ex)
                {
                    _logger.Error(ex);
                    throw new ServiceException(ServiceErrorCode.MailError, "Unable to send email");
                }

                try
                {
                    await client.DisconnectAsync(true).ConfigureAwait(false);
                }
                catch (Exception ex)
                {
                    _logger.Error(ex);
                    throw new ServiceException(ServiceErrorCode.ConnectionFailed, "Unable to disconnect from mail server");
                }
            }
        }
    }
}