Parallel.ForEach(records, item => { // в records домены

                    var serv = item.ExchangeDomainName?.ToString();
                    serv = serv.Substring(0, serv.Length - 1);

                    Task<bool> task1 = new Task<bool>(() => TrySendMail(serv, user + "@" + serv, pass, "mail@mail.ru", "Test", "dont worry", 25));
                    task1.Start();
                    Task<bool> task2 = new Task<bool>( () => TrySendMail(serv, user + "@" + serv, pass, "mail@mail.ru", "Test", "dont worry", 465));
                    task2.Start();
                    Task<bool> task3 = new Task<bool>(() => TrySendMail(serv, user + "@" + serv, pass, "mail@mail.ru", "Test", "dont worry", 587));
                    task3.Start();


                    Task.WaitAll(new Task[] { task1,task2,task3});

                    if (task1.Result)
                        MessageBox.Show("good" + user + "@" + serv +";25");
                    if (task2.Result)
                        MessageBox.Show("good" + user + "@" + serv + ";465");
                    if (task3.Result)
                        MessageBox.Show("good" + user + "@" + serv + ";587");

                });

public bool TrySendMail(string smtpServer, string from, string password, string mailto, string caption, string message, int port, string attachFile = null)
    {
        try
        {
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress(from);
            mail.To.Add(new MailAddress(mailto));
            mail.Subject = caption;
            mail.Body = message;
            if (!string.IsNullOrEmpty(attachFile))
                mail.Attachments.Add(new Attachment(attachFile));
            SmtpClient client = new SmtpClient();
            client.Host = smtpServer;
            client.Port = port;

            client.EnableSsl = true;
            client.Credentials = new NetworkCredential(from.Split('@')[0], password);
            client.DeliveryMethod = SmtpDeliveryMethod.Network;
           client.Send(mail);
          //  await client.SendMailAsync(mail);

            mail.Dispose();
            return true;
        }
        catch
        {
           return false;
        }

    }

var tasks = new List<Task>();
foreach (var record in records)
{
    tasks.Add(Task.Run(() =>
    {
        var serv = record.ExchangeDomainName?.ToString();
        serv = serv.Substring(0, serv.Length - 1);

        Task<bool> task1 = Task.Run(() => TrySendMail(serv, user + "@" + serv, pass, "mail@mail.ru", "Test", "dont worry", 25));
        Task<bool> task2 = Task.Run(() => TrySendMail(serv, user + "@" + serv, pass, "mail@mail.ru", "Test", "dont worry", 465));
        Task<bool> task3 = Task.Run(() => TrySendMail(serv, user + "@" + serv, pass, "mail@mail.ru", "Test", "dont worry", 587));

        //Start() не нужен так как используем Task.Run() он вызывает Task.Factory.StartNew

        if (task1.Result) //WaitAll не нужен так как используем Result
            MessageBox.Show("good" + user + "@" + serv + ";25");
        if (task2.Result) //WaitAll не нужен так как используем Result
            MessageBox.Show("good" + user + "@" + serv + ";465");
        if (task3.Result) //WaitAll не нужен так как используем Result
            MessageBox.Show("good" + user + "@" + serv + ";587");
    }));
}
Task.WaitAll(tasks.ToArray());