How to Use Telnet To Check SMTP Service

Posted on by Jay Allen
Reading Time: 3 minutes

One of the most common e-mail issues we encounter in our support department is customers who are traveling and are encountering problems trying to send e-mail.

Quite often this is the result of connecting to a new network that you haven’t used to send e-mail before, like an open wireless network in a coffee shop, library, or other public location. Every network has rules, and in some cases networks have rules about what outbound e-mail servers (SMTP) you can connect to and use.

The main reason for such a rule or block is to protect that network from being blocked as a spam source (junk e-mail). People who send spam are traced by their IP address, so they jeopardize the reputation of the network they are using to connect to the internet.

By limiting SMTP connections on a network to only one acceptable SMTP server that network can be more closely controlled and, in theory, protected from being labeled as a spam source. These changes can often be made suddenly and without warning to the people using the affected network.

Please note that even if you are having problems sending email from a home or work/office network you can still use the following commands to test your connection to your usual SMTP server.

Windows Users
If you are using Windows Vista or Windows 7 you may need to enable the Telnet client prior to following the rest of the instructions listed below. If you are unable to run Telnet at the command prompt you will need to follow these extra steps:
1. Click on Start.
2. Click on Control Panel.
3. Click on Programs and Features.
4. Click on Turn Windows Features On and Off.
5. Check the box next to Telnet Client and click OK.
You should now be able to run the telnet command from the command prompt and proceed with the instructions below.

Click on Start and then Run. In the run box that appears type in the word command and press enter. cmd will also work for many versions of windows. This will open a command prompt window.

Skip to the “Type in the following command” section below.

Mac OS X Users
Open the Finder and go to Applications. Open the Utilities folder and double-click on the Terminal application.

Type in the following command:
telnet 25
This will connect to the mail server on port 25, which is the port used to send e-mail.

If your connection is successful you should see a response from the SMTP server similar to the one below:
telnet 25

Trying [IP Address of mail server]...
Connected to
Escape character is '^]'. ESMTP Exim 4.69 #1 Mon, 29 Jun 2009 11:27:42 -0400
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.

This is an example of an expected response, meaning that your computer is able to connect to the target SMTP server on port 25 successfully. What you see here is the SMTP server greeting, an announced message that the server displays whenever a connection is made to the SMTP port. When you use an email client like Thunderbird or Outlook the server greeting is not displayed.

If your computer were unable to connect you would receive an error message after your telnet command like Server Timeout, Could Not Reach Host, or a similar message. This indicates that there is a block in place on the network you are connected to.

If you are connecting from home or your office you will need to get in touch with either your internet service provider or a local network administrator to determine your available options. Most networks that block access to off-network SMTP servers allow you to use their e-mail server to send messages from any domain address.

Because these blocks can be added at any time your ISP’s support department might not be aware of the block when you contact them. You may need to ask them to run the telnet test from their end or contact one of their internal network specialists for more information.


Liquid Web’s Heroic Support is always available to assist customers with this or any other issue. If you need our assistance please contact us:
Toll Free 1.800.580.4985
International 517.322.0434

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About the Author: Jay Allen

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