As with most things more than one way exists to skin this cat. The two most common ways I discovered (both of which have some limitations and problems) were to either use HTTP REDIRECT in IIS Manager, or to use a custom 403 error page.
The simplest and most elegant solution I found was to create a one-line default.asp file. Of course you have to have ASP installed/enabled on the server and default.asp needs to be in (preferably alone or at the top of) your default documents list.
Wait no longer - all you need in the default.asp file is:
<% Response.Redirect "https://mail.mydomain.com/owa" %>That's it. It's that simple!
Now when your users access (http://)mail.mydomain.com they will magically and instantly be redirected to the secure and correct location of https://mail.mydomain.com/owa.