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How to Test an Email Server with the Telnet Client

Summary: Telnet Client testing on an Email Server.

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Article Summary: This article provides information on using the Telnet client to test a mail server.


Table of Contents:

1. Install the Telnet client
2. Test the connection and send an email


1. Install the Telnet client


Testing a connection to a mail server with Telnet can yield valuable information when troubleshooting a mail-flow issue. For instance, if the connection from the local server is being denied, there will typically be a human-readable error message displayed within the Telnet session. If the error description doesn't provide sufficient information to troubleshoot the issue, there will typically be an SMTP error code provided as well. Researching this error code should provide a wealth of useful information.

Before you can use the Telnet client to test connectivity, it must first be installed. Below are instructions for installing the Telnet client in Windows Server 2008 and 2008 R2.

  1. Launch Server Manager.
  2. In the left pane, click Features.
  3. In the right pane, click Add Features.
  4. Check the box labeled Telnet Client and click Next.
    Note: Do not check the Telnet Server box. This feature is used to allow other machines to connect to this server using Telnet.
  5. Click Install.
  6. Once the installation is complete, click Close. It should not be necessary to reboot the server.


Note: While it is recommended to perform this test from a mail server (in other words, install the Telnet client on your local mail server and use it to connect to a remote one) in order to closely simulate real-world email delivery, it is also possible to install the Telnet client on a Windows client OS, such as Windows 7, by using the Programs and Features Control Panel applet.



2. Test the connection and send an email


Once the Telnet client has been installed, follow the steps below to test email delivery.

  1. Open a command prompt.
  2. Type the following command: telnet <mail server name or IP> 25
    • You should receive output similar to the following:
      220 ESMTP
      NOTE: Once the telnet session is established, the Backspace and Delete keys do not work. If a typo is made, press Enter and retry the command. In some cases, it may be necessary to close the telnet session and reconnect.
    • If you receive an error similar to Could not open connection to the host, on port 25: Connect failed, this indicates that your machine could not establish a telnet session with the destination server using port 25. If you initially attempted to connect using the destination server's name, try to connect using its IP address instead. If this is successful, you likely have a DNS issue. If you are unable to connect using the destination server's name or IP address, port 25 is being blocked somewhere between your machine and the destination server. The issue is probably due to a firewall on the destination server's end, but it can also be caused by a network misconfiguration or firewall on your end.
    • If you receive output similar to the following:
      554 Your connection has been blocked due to low sender reputation...
      Connection to host lost.

      The public IP address associated with your mail server has developed a bad reputation. If you recently acquired this IP address, its previous owner likely sent spam from it, and your best option may be to request a different address from your ISP. If this is not feasible, there are other options, but there is not always a quick or easy solution to this issue. The error message may include additional instructions for resolving the issue. If not, this blog entry has a number of suggestions. Unfortunately, it may simply be a matter of waiting for your address's reputation to improve, and this can take considerable time.
  3. Type: EHLO <mail server internet name>
        Replace <mail server internet name> with your mail server's public fully qualified domain name - for instance,
  4. Type: mail from: <> and press Enter.
  5. Type: rcpt to: <> and press Enter.
  6. Type: data and press Enter.
  7. Type: Subject: <a subject here> and press Enter.
  8. Type: This is a test message sent from telnet. and press Enter.
  9. Type:  .   and press Enter
        The message should be accepted for delivery.


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Article Properties

Affected Product

Servers, PowerEdge

Last Published Date

11 Mar 2021



Article Type