Troubleshooting SMTP Server Connections with Telnet
We get lots of support calls from customers who are not able to connect to their SMTP server with our email components. In just about every case it is not a problem related to our components but a problem with the computer itself. Port 25 can be blocked by firewalls, anti virus software and ISPs. So the first thing we have the customer do is to verify that they can connect with telnet.
Using telnet is simple, just open a command window and type:
telnet your.mail.server 25
and press return. This will initiate a connection with your.mail.server on port 25. If the command is successful, you will get a response from the SMTP server. Here is an example response, although yours will vary:
220 107241-WWW1 SMTP Express Pro ESMTP service ready.
You can enter QUIT followed by return to terminate the connection to the SMTP server.
If you did not receive a response from the SMTP server then your computer is being blocked somewhere along the line.
To further diagnose the problem, you can try disabling anti virus software and system firewalls to see if that helps. If that is the source of the problem, you should be able to configure your AV or firewall software to allow outbound traffic on port 25.
If you have disabled AV and firewall software and still can not connect to your email server with telnet, you might want to try connecting to your ISP’s email server as well as any others that you might know of such as smtp.gmail.com. If you can connect to your ISP’s email server but no others then it is likely that your ISP is blocking port 25 and you may have to configure your mail server to listen on an alternate port.