为什么smtpclient在xp/vista/win7上的行为会不同?

我正在使用以下代码,在Vista/Win7上似乎每次都能很好地工作。

private void SendEmail(string subject, string body, string attach)
{
    using (MailMessage message = new MailMessage("username@gmail.com", "username@gmail.com", 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("username@gmail.com", "password"),
            EnableSsl = true,
            DeliveryMethod = SmtpDeliveryMethod.Network
        };

        client.Send(message);
    }
}

但是在Windows XP上,我得到了:

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

我检查过,Windows防火墙已完全禁用…