Iis – Configure IIS SMTP server to forward to more than one address

Yes, the SMTP service in IIS is capable of full end-to-end delivery. In order for it to function this way, the following requirements must be met:

• The server needs to be on a routable IP address (ie not an IP address in one of the reserved private ranges)
• The DNS settings on the server need to be configured properly so it can resolve MX records of the recipient domains properly
• You need to clear out the "smart host" setting. Find this by right-clicking on the SMTP server in the IIS Admin console, select Properties, select the "Delivery" tab, click the "Advanced" button, and you will see the space for entering a smart host.

Rather than poking at a black box and wondering why it won't work, install "Network Monitor" or Wireshark on the SMTP relay machine and sniff some SMTP traffic. You'll find out in about 30 seconds why your mail is being rejected. That's going to be your path of least resistance. The box wants to tell you what's wrong, you just need to look.

Your "network engineer" should've reached for the sniffer earlier, IMO. Sniffing the traffic on the wire seems "taboo" to some people, but it's often the best way to get to the bottom of a problem in a human-readable protocol like SMTP.