We are having and issue with sending some emails to domains outside of our network. We fixed a reverse dns issue and it did not resolve all of the issues.

We then realized that we could not connect via telnet over port 25 to these external mail sites (telnet mail.yourhost.com 25). We can connect from other servers to these external sites via telnet just not from the mail server.

What might be causing telnet not to work from the mail server?

Server - Windows 2003

Email - Exchange 2003


There was a firewall rule setup for RPC over HTTP that was causing the issue. Once the rule was removed everything was restored to normal.

  • re: "update: there are no firewall rules setup blocking this server or port 25" - have you confirmed that your ISP/host is not blocking outgoing connections, or just verified your own firewall arrangements? Jun 29, 2009 at 17:16

7 Answers 7


The most obvious answer is a firewall somewhere - have you checked the software filewall on the mailserver, and any hardware firewalls/routers in the way?

Can you telnet to other ports on the external severs? Can you ping/traceroute them?


I suspect some form of firewall is blocking your outgoing SMTP (port 25) connections.

If none of your firewall arrangements are blocking these connections, then it may be your ISP or server host that is. Many ISPs do not allow direct outgoing connections on port 25, instead insisting that all mail traffic go through their smarthost(s), as a spam prevention/detection measure. If you are a small company operating off a DSL line this this could well be your problem.

  • check that remote servers listen on port 25
  • check firewall settings on both sites

If there is an anti-malware app installed on the Exchange server, ensure it isn't capable of blocking outbound :25TCP. (Been there, done that... thanks, McAfee :p)


Most likely case is firewall block either at your end or there end. If its not working for ANY domain mail server then most likely at your end. If its hit and miss for most domains and there is a possiblity that your mail server's public IP is on blocked list of some the the anti-spam systems.

Check the status of your public ip on following websites.

IronPort Senderbase IronMail TrustedSource SpamHaus


antivirus? mcafee blocks this as the "prevent mass mailing worms" rule.


Warning! Science Content!

You're not actually telnetting to port 25 when you do this - you're in fact opening a TCP connection to port 25 in exactly the same way that any client or server would. You're just making the connection manually. Telnet is simply doing the same thing on port 23.

As a result, there's nothing preventing Telnet from creating the connection any more than there is preventing Outlook from creating the connection, although the Windows Vista firewall does work on a per-application basis.

Which is why everyone is answering your question with "There's a firewall blocking you."

That firewall, coincidentally, doesn't have to exist on your computer. It could exist at your ISP (or anyone with a router between yourself and your destination!). If I were in control of the networking at your ISP, I would block outgoing connections on port 25 to any host that isn't our mail servers. This would make your ISP a good net citizen by preventing massive amounts of spam from coming from our network courtesy of various malware applications. ISPs that do this typically request that users with their own (legitimate!) mail servers use their outgoing SMTP servers as "smart hosts". And they probably also rate-limit their connections. And then block anyone abusing it and disconnect them.

Another possible source of the problem is that the recipient's SMTP server could be blocking you. You would know this is the problem when you can connect to some hosts on port 25, but not others. The only way you can find out the reason why they're blocking you (unless their server gives that reason in the error message), is by asking them nicely. After you switch Outlook's outgoing mail server to your ISP's that is.

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