When you try to send or receive messages by using an Exchange server client, you may receive an error message. When you try to send a message to your Simple Mail Transfer Protocol (SMTP) server, you may receive the following error message: Similarly, when you start the e-mail client or try to receive a message from your Post Office Protocol version 3 (POP3) server, you may receive the following error:

The following workaround should be performed on only the Exchange Server. If you are receiving one of the error messages that is listed in the "Symptoms" section and if you are not on a corporate network, you should view the following Microsoft Knowledge Base article for information about how to troubleshoot this issue in Microsoft Outlook and Microsoft Outlook Express: Warning This workaround may make a computer or a network more vulnerable to attack by malicious users or by malicious software such as viruses. We do not recommend this workaround but are providing this information so that you can implement this workaround at your own discretion. Use this workaround at your own risk.

To work around this behavior, make ports 110 and 25 available. By default, ports 110 and 25 are available when the Exchange Server services start. The ports 110 for POP3, 119 for Network News Transfer Protocol (NNTP), and 143 for Internet Message Access Protocol (IMAP) are automatically opened and made available on the Exchange Server computer when the Information Store service starts. Port 25 is automatically made available when the Internet Mail Service starts. If these services do not start, the ports are not available.

If the services are running, you can verify that these ports are listening (open) on the server by using the Netstat utility:

1. Open a command prompt on the Exchange Server computer. You have to modify the command prompt properties, because the data scrolls off the screen. Open the command prompt, right-click the upper-left corner of the window, and then click Properties. Click the Layout tab, and then change the screen buffer size to 1,000. Apply the change. You are prompted to modify either the current window or the shortcut that you used to start the command prompt. You can modify the shortcut for future convenience.
2. At the command prompt, type netstat -an. Scroll back to the beginning of the data output, and look for the following entries.

   Proto Local Address          Foreign Address        State  TCP    0.0.0.0:25             0.0.0.0:0              LISTENING  TCP    0.0.0.0:110            0.0.0.0:0              LISTENING      

If these entries are present, Exchange Server is listening on ports 110 and 25 on all interfaces. This is typical. If these ports are not available, you may have to troubleshoot the following services or systems more. The following general causes may "hold" these ports, or make them unavailable:

• The Exchange Server information store service is not running, or the POP3 protocol is not enabled and listening to port 110. Verify that the information store service is running. For more information about troubleshooting the POP3 protocol, click the following article number to view the article in the Microsoft Knowledge Base:
  161118  (http://support.microsoft.com/kb/161118/ ) Troubleshooting PO161118 connections to Exchange Server
• The Internet Mail Service is not running or is not listening on port 25. Verify that the Internet Mail Service is running. For more information about testing the Internet Mail Service and port 25, click the following article number to view the article in the Microsoft Knowledge Base:
  153119  (http://support.microsoft.com/kb/153119/ ) Telnet to port 25 to test SMTP communication
• A proxy server located between the client and the server may be blocking access to these ports to the Exchange Server computer. For more information about configuring Exchange Server through a proxy server, click the following article number to view the article in the Microsoft Knowledge Base:
  176466  (http://support.microsoft.com/kb/176466/ ) TCP ports and Microsoft Exchange: In-depth discussion
• Another program, such as virus-scanning software, has reassigned port 25 to another port. Disable all services that are not essential, including antivirus software, and then restart the server.