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Cannot connect to port 25 on a Mail server


Cannot connect to port 25 on an MS Exchange server to use its SMTP service. A telnet test gave the following error:

telnet xx.xx.xx.xx 25
Connecting To xx.xx.xx.xx…Could not open connection to the host, on port 25: Connect failed

Background & Analysis:

One of my batch scripts used blat to send emails. However, I could not use the SMTP service on the Exchange Server. After liaising with the Exchange and Network administrators, we determined the following:

  • There was no firewall between my machine and the Exchange server and a tracert completed quickly in just 2 hops.
  • There was no firewall on the Exchange server.
  • The firewall on my machine was turned off.
  • The SMTP service was listening at port 25 on the Exchange server and was functioning properly.


The McAfee Anti-Virus log on my machine had the following entry:

16/12/2010 12:41:02 PM Blocked by port blocking rule C:\WINDOWS\system32\telnet.exe Anti-virus Standard Protection:Prevent mass mailing worms from sending mail xx.xx.xx.xx:25

So, it was the McAfee anti-virus software that blocked outbound connections to port 25. To resolve this problem, you can add an exception to the anti-virus policy that permits your mail program (in my case, it was blat) to initiate connections to port 25 (recommended) or you may disable the anti-virus (not recommended).

Root Cause:

The Anti-virus software on the source machine had a policy enabled which prevented outbound connections to port 25.


(1) The solution above describes a successful problem-solving experience and may not be applicable to other problems with similar symptoms.
(2) Your rating of this post will be much appreciated. Also, feel free to leave comments.


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