Sending email through GMail and .NET 4

Question

Are there any problems with .NET 4 when sending emails? I had a .NET 3.5 solution, and it worked, and then ported the solution to .NET 4, and it does not work; nothing changed!

Notes: I get this exception:

System.Net.Mail.SmtpException: SMTP server requires a secure connection, or the client failed authentication. Server response: 5.5.1 Authentication required. Learn more at at System.Net.Mail.MailCommand.CheckResponse (SmtpStatusCode statusCode, string response)
at System.Net.Mail.MailCommand.Send (SmtpConnection connection, Byte [] command, line out)
at System.Net.Mail.SmtpTransport .SendMail (MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException & exception)
in System.Net.Mail.SmtpClient.Send (MailMessage message)
when ...

And this is my code:

public static void SendEmail(
    this Email email)
{
    if (email.MailingSettings == null) throw new ArgumentNullException("email.MailingSettings", "specify email.MailingSettings");

    var settings = email.MailingSettings;

    if (string.IsNullOrEmpty(email.Sender)) throw new ArgumentException("email.Sender", "specify a sender");
    if (email.Recipients == null) throw new ArgumentNullException("email.Recipients", "specify at least a recipient");
    if (email.Recipients.Length == 0) throw new ArgumentNullException("email.Recipients", "specify at least a recipient");
    if (string.IsNullOrEmpty(settings.SMTPHost)) throw new ArgumentException("email.SMTPHost", "specify email.SMTPHost");

    var mail = new MailMessage();

    if (string.IsNullOrEmpty(email.SenderDisplayName))
        mail.From = new MailAddress(email.Sender);
    else
        mail.From = new MailAddress(email.Sender, email.SenderDisplayName);

    if (!string.IsNullOrEmpty(email.ReplyTo))
    {
        if (string.IsNullOrEmpty(email.ReplyToDisplayName))
        {
            mail.ReplyToList.Add(new MailAddress(email.ReplyTo));
        }
        else
        {
            mail.ReplyToList.Add(new MailAddress(email.ReplyTo, email.ReplyToDisplayName));
        }
    }

    foreach (string recipient in email.Recipients)
        mail.To.Add(new MailAddress(recipient));

    mail.Subject = email.Subject;
    mail.Body = email.Body;
    mail.IsBodyHtml = settings.IsBodyHtml;

    if (email.CC != null && email.CC.Length > 0)
        foreach (string cci in email.CC)
            mail.CC.Add(cci);

    if (email.BCC != null && email.BCC.Length > 0)
        foreach (string bcci in email.BCC)
            mail.Bcc.Add(bcci);

    if (email.Attachments != null)
        foreach (var ifn in email.Attachments)
            mail.Attachments.Add(
                new Attachment(
                    new MemoryStream(
                        ifn.Content),
                    ifn.FileName));

    var smtpPort = settings.SMTPPort > 0 ? settings.SMTPPort : 25;

    var smtpClient = new SmtpClient(settings.SMTPHost, smtpPort);
    smtpClient.EnableSsl = settings.EnableSsl;

    if (!string.IsNullOrEmpty(settings.SMTPUser))
    {
        smtpClient.UseDefaultCredentials = false;

        var smtpPassword = settings.SMTPPassword == null ? string.Empty : settings.SMTPPassword;

        NetworkCredential netCred;
        if (string.IsNullOrEmpty(settings.SMTPDomain))
            netCred = new NetworkCredential(settings.SMTPUser, smtpPassword);
        else
            netCred = new NetworkCredential(settings.SMTPUser, smtpPassword, settings.SMTPDomain);

        smtpClient.Credentials = netCred;
    }
    else
        smtpClient.Credentials = CredentialCache.DefaultNetworkCredentials;

    smtpClient.Timeout = 180 * 1000;
    smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;

    ServicePointManager.ServerCertificateValidationCallback =
        delegate(object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        { return true; };

    smtpClient.Send(mail);
}

Answer 1

. GMail ( , ).

Answer 2

, , google, Application Specific-password.