Why would SmtpClient behave differently on XP/Vista/Win7

Question

I'm using the following code which appears to work perfectly every time on Vista/Win7.

private void SendEmail(string subject, string body, string attach)
{
    using (MailMessage message = new MailMessage("[email protected]", "[email protected]", subject, body))
    {
        message.IsBodyHtml = true;

        if (!string.IsNullOrEmpty(attach))
        {
            Attachment attached = new Attachment(attach);
            message.Attachments.Add(attached);
        }

        SmtpClient client = new SmtpClient("smtp.gmail.com", 587)
        {
            Credentials = new NetworkCredential("[email protected]", "password"),
            EnableSsl = true,
            DeliveryMethod = SmtpDeliveryMethod.Network
        };

        client.Send(message);
    }
}

However on Windows XP I'm getting:

No connection could be made because the target machine actively refuses it

I've checked and Windows firewall is completely disabled...

Answer 1

Try from the Windows machine the following:

1. windows key + r
2. Type cmd
3. Type telnet smtp.gmail.com 587

If it says connection refused or similar then it's a firewall or network problem, unrelated with the code.

Answer 2

Hard to say if this is it, but we had that problem at one point, and it was an antivirus utility that was the culprit.

Answer 3

Are you using the same version of System.Net.Mail on all three systems ?

Also, could be related to the Windows Firewall blocking connections (or some other firewall).

Answer 4

I doubt this has anything to do with the OS, that type of exception is usually bubbled up from inner ones. Trap the exception and have a look into the inner exceptions and see what the real problem is.

However, this sort of problem is usually a firewall blockage, the remote smtp server is blocking incoming requests or your machine is blocking outgoing requests on port 25.