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Regarding the SMTP service that is including in Microsoft's Internet Information Services (IIS) 6.0.

I have it in my mind that the built-in SMTP service is not capable of routing outbound messages, except to spcific domains that have to be manually specified. However, I've just had a conversation with someone that has challenged my understanding. I've looked at various articles on the web and they are only confusing me more.

So, can anyone categorically state whether or not the IIS SMTP service can be used as a general purpose relay for outbound mail? For example, can I set up a web site that uses it to send outbound mail and the SMTP service will go out and look up the MX records for the recipient and route the email?

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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.
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  • "The server needs to be on a routable IP address (ie not an IP address in one of the reserved private ranges" - well, it depends. If the clients or the server itself are all on the private network, this is not absolutely necessary.
    – splattne
    Jul 24, 2009 at 11:05
  • Good point. In general though not having the server on a routable IP causes many problems (unless NAT is properly configured, which effectively works as if it did) - Reverse DNS lookups will fail etc.
    – Bork Blatt
    Jul 24, 2009 at 11:10
  • Does the firewall have to have port 25 opened up for both outbound and inbound ... if we're only caring about sending email out (eg. sign up emails, error emails, etc) ?
    – Pure.Krome
    Oct 15, 2009 at 5:29
  • Nope, if you are only sending e-mail you only need port 25 opened for outbound traffic.
    – Bork Blatt
    Oct 28, 2009 at 10:21
  • A "routable IP address" means a public IP address, right? If so, why is that necessary if NAT enables the server to reach out to the Internet and you only need to do outbound emails?
    – TheStoryCoder
    Sep 21, 2018 at 13:13
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Yes, the IIS SMTP can be used as normal SMTP server.

If you're experiencing problems, try to look for IP restrictions for Relaying on the SMTP server in the "Connections" window of the SMTP configuration. You could also check if a firewall is blocking TCP port 25 traffic from that server.

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  • No problems, other than in my head ;-)
    – Tim Long
    Jul 24, 2009 at 10:58

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