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);
        var mailFrom = $"{user}@{serv}";
        var ports = new[] { 25, 465, 587 };

        var taskForPorts = new Dictionary<int, Task<bool>>();
        foreach (var port in ports) //Ставим задачи на проверку отправки писем на конкретном порту
        {
            taskForPorts.Add(port, TrySendMailAsync(serv, mailFrom, pass, "mail@mail.ru", "Test", "dont worry", port));
        }

        Task.WaitAll(taskForPorts.Values.ToArray()); //Ожидаем завершения задач на проверку отправки писем на портах одного домена;

        foreach (var port in ports)
        {
            if (taskForPorts[port].Result)
                MessageBox.Show($"good {mailFrom};{port}"); //Тут точно нужна ";"?
        }
    }));
}
Task.WaitAll(tasks.ToArray());

public async Task<bool> TrySendMailAsync(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;
        await client.SendMailAsync(mail);

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