* Open Computer Management
* Expand Services and Applications, expand Internet Information Service
* Right click Default SMTP Virtual Server and click Properties
* Click the Access tab
* Click the Authentication button and make sure Anonymous Access and Integrated Windows Authentication is enabled.
* Click the Relay button and make sure Allow all computers which successfully… is enabled and Only the list below is selected.
First of all, Authentication and Relay is not the same thing. We use the Authentication button to specify which authentications methods are allowed for users and other SMTP servers. So enabling Anonymous here is not a security issue, in fact, it’s required if we want our server to be able to receive emails from other servers on Internet (I doubt you want to tell all administrators of email servers on Internet how they should logon to yours). We also need Windows Authentication so the email clients can authenticate to the server and be able to relay (send emails).
As Relay Restrictions we selected Only the list below because we do not want to be used by spammers to send emails. But we never specified any computers. That is valid, because we wants our clients to always use the username and password to authenticate, no matter where they are.
If you want users to only be allowed to relay if they are on a private network, then you can uncheck Windows Authentication as allowed authentication method, and specify the IP range for your network in the Relay Restrictions window.