Using URL Rewrite I could redirect mydomain.tld to https://mydomain.tld,
but failed to redirect mail.mydomain.tld to https://mail.mydomain.tld,
I used the same way to configure the URL Rewrite for both,
and each one of them has its dedicated IP and its dedicated SSL certificate.
An example of the followed method is here: http://www.jppinto.com/2010/03/automatically-redirect-http-requests-to-https-on-iis7-using-url-rewrite-2-0/