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I am having trouble with our Exchange server at the moment and cannot work out what is wrong. Basically I can receive internal mail on the domain between exchange users but nothing from an external address comes through at all.

Port 25 is open

Does anyone know what could be wrong?

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Check your firewall isn't blocking traffic on port 25, your ISP isn't blocking traffic on port 25, your MX records are spiffing, try telneting to your server using an external connection (think at home or a 3G dongle). – Ben Pilbrow Sep 21 '11 at 19:12

There are tons of possible reasons for your mail server to not be able to receive external messages.

  • Your server could be having troubles. Try telneting to its TCP port 25 from your internal network and see if it answers; also try sending a message to it, either manually (if you know some SMTP) or using any SMTP mail client.
  • Your firewall could be blocking traffic. Try telneting to port 25 of your server's external (public) IP address.
  • Your public DNS settings could be wrong. Check if you can resolve your public domain's MX records, what does it point to, and if that name actually maps to your server's public IP address.
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In addition to other answers, make sure that reverse DNS for your IP address is working, as some hosts check on that before sending mail.

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