You can make it easier on your webmail users by automatically redirecting requests for http://owa.mailserver.com to https://owa.mailserver.com pretty easily using IIS Manager.
1) Open IIS Manager, expand Sites, and then click Default Web Site.
2) Double-click HTTP Redirect and select the Redirect requests to this destination check box. Enter the full destination URL (i.e. https://owa.mailserver.com/owa)
3) Under Redirect Behavior, select the Only redirect requests to content in this directory, select Found (302) as the status code, and click Apply.
4) Click on Default Web Site to take you back to the features view, and double-click SSL Settings. Uncheck the Require SSL box.
5) Open command prompt as administrator and run IISRESET.
Open a new browser and test going to http://mail.mailserver.com to see if it redirects you to https://mail.mailserver.com/owa automatically.
NOTE: I have seen issues with these changes not taking effect immediately, even if you follow the above 5 steps to a tee. The problem lies with the web.config file not updating correctly. If you’re sure you’ve done the above steps correctly and it’s still not redirecting properly, browse to C:\inetpub\wwwroot. Rename the web.config file to web.config.old. Run IISRESET and try it again. The file will recreate/rebuild itself and redirection should then work. It also appears that in some instances, applying any of the Exchange Cumulative Updates, will break the redirection making you have to re-perform this process.