Question

I am able to ping smtp.mail.yahoo.com from my system but when i send email from following code using yahoo address it gives error transport failed to connect to server.
The same code successfully sends the email from gmail account.

I am using port 465 for yahoo.

    MailMessage oMsg = new MailMessage();
    oMsg.From = from.Text;
    oMsg.To = to.Text;
    oMsg.Subject = "Hi";
    oMsg.BodyFormat = MailFormat.Html;
    oMsg.Body = msg.Text;


    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserverport", port);
    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserver", host);
    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusing", 2);

    WebProxy proxy = WebProxy.GetDefaultProxy();
    if (proxy.Address != null)
    {
        oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/urlproxyserver", proxy.Address.Host);
        oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/proxyserverport", proxy.Address.Port);
    }
    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpusessl", true);
    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1");
    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername",from.Text);
    oMsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", pass.Text);


    // ADD AN ATTACHMENT.

    /*  MailAttachment oAttch = new MailAttachment(path+ "\\Image.bmp", MailEncoding.Base64);

    oMsg.Attachments.Add(oAttch);*/

    SmtpMail.SmtpServer.Insert(0,host);
    if (proxy.Address != null)
        MessageBox.Show("Sending via proxy settings: " + proxy.Address.ToString());
    try
    {                
        SmtpMail.Send(oMsg);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    oMsg = null;

Any ideas why this error occurs?

Answer 1

Being able (or not) to ping a host does not say anything about whether you will be able to connect to a particular service on it. For that, you need to try to actually establish a connection. (And of course, the fact that you can establish a connection does not necessarily imply that the service in question is working properly.)

Usually, it’s a good idea to use telnet to try connecting to the remote host on the port in question. The syntax on the command line is simply telnet host.fqdn.example.com portnumber. This will tell you if there is anything at all at the other end of the pipe responding to connection attempts, which is a first step in determining where the problem is.

Second, it’s usually a good idea to trim the code to the minimal version that exhibits the problematic behavior, and include the full code to show the problematic behavior. You are using a number of variables in your code which we really know nothing about.

Some ISPs block outgoing connections to the SMTP ports on hosts other than their own mail servers, to reduce the amount of outgoing spam. Maybe there is a typo in the value in host? Maybe you are inadvertantly using some unexpected MailMessage implementation? And so on.

That said, I would definitely first try to connect to the mail server in question manually, through a proxy if you are using one to connect using that code. If that doesn’t work either, then your problem at least has nothing to do with the code in the question, and you can look elsewhere (in which case one possible candidate would be ISP filters; maybe they have a list of allowed external SMTP hosts and Yahoo’s isn’t on it?).