This happens because the SSL settings on the Default Web Site and all subdirectories are configured to require SSL.
In multi-server environments where load balancers are used, HTTP -> HTTPS redirection is normally performed on the load balancer. Most load balancers, such as KEMP or F5, automatically configure it when you deploy one of their Exchange server templates.
In single Exchange environments without a load balancer, you must configure redirection directly on the server itself. The steps below explain how to do this. I've used this method since Exchange 2007 and it works perfectly 100% of the time. Other methods I've seen on the Internet sometimes cause routing errors.
- Open Internet Information Services (IIS) Manager on the Exchange 2016 or Exchange client access server and navigate to Sites > Default Web Site.
- Double-click Error Pages and add a new custom error page for status code 403.4 that responds with a 302 redirect to https://mail.contoso.com/owa.
- Special note for Exchange 2016 - The Exchange team did a little number in the web.config file on Exchange servers to "improve performance", but it removes the custom error page behavior. So you'll need to do the following:
- Open the C:\inetpub\wwwroot\web.config file in Notepad.
- Remove the following line, and then save and close Notepad:
<remove name="CustomErrorModule" />
Note that this web.config edit will need to be made after every Exchange 2016 CU installation since setup overwrites this file.
That will handle the HTTP 403.4 - Forbidden: SSL is required error behavior in the browser. All that's left is to handle what happens when a user enters https://mail.contoso.com.
- Using Notepad, create a web page called default.htm in the C:\inetpub\wwwroot folder of the Exchange 2016 or client access server. Add the following three lines:
<meta http-equiv="REFRESH" content="0;url=/owa">
- Save and close Notepad, and then test redirection.