Too bad we need to use a workaround; here is the easiest way I've found to do it:
I use enom.com as registrar for my domains. In my case, I just set up a "URL Redirect" for webmail.yourdomain.com that points at https//mail.yourdomain.com where mail.yourdomain.com is, of course, the public hostname/IP address of the SM server.
Inside the LAN, I use Windows DNS services to point users back at the LAN address of the SM server for mail.yourdomain.com (the enom DNS URL Redirect still works as expected). Browser requests for hostnames that are not specifically defined on the Windows DNS server are, of course, forwarded to external Internet name servers.
The redirect works about 99.9% of the time. Usually it's a browser problem where it doesn't work, and clearing the cache usually fixes it.