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I'm configuring the SMTP service on a new Windows Server 2016 build. I've installed the required software, accessed the Admin tool and set up access and relay control.

This server will just be handling outbound mail for status messages from printers, backup systems, etc. There will be no mail coming in from outside the network.

I'm trying to set up the ISP's mail relay as a smart host, but I'm having trouble. I've entered the fully qualified domain name for the server, left the Masquerade name blank, and added the IP address of the smart host (The address I've used is the address of a known SMTP relay on my test network. It will change in production)

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The problem is this: if I click the Check DNS button I get a message saying the domain name is valid. If I then click the OK button, I get a message saying the domain name is not valid.

What have I missed?

  • Try without the dot at the end. Also the name has no MX record but only an A record. This is valid but some software may make an MX mandatory... – Patrick Mevzek Apr 21 '18 at 1:49
  • @PatrickMevzek Thanbks for the comment. I'd already tried the dot variation. There should be an MX record too, but the fix proved to be different. See my answer below. – Redd Herring Apr 21 '18 at 1:50
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This proved to be a relatively simple fix.

The "Domain name is not valid" message originates from the Smart Host field, not the FQDN field. I'd entered the IP address of the smart host I wanted to use, but the system wanted the FQDN.

Simply changing the smart host address from an IP address to a name it could resolve fixed the problem.

I have no idea why!

  • When entering an ip address in the smart host field the ip address must be enclosed in brackets. This has been the case since at least Windows 2000. – joeqwerty Apr 21 '18 at 1:57

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