The solution to your question is as follows:
- Create http://webmail.domain.com as a new server (a virtual server)
- Make sure is serves over port 80 only
- Make sure it does not use the SSL cert
- Create https://webmail.domain.com as a new server
- Make sure is serves over port 443 only
- Make sure it uses the SSL cert.
- Go back to the http://webmail.domain.com, create a alias
- Type: Redirect Match
- Pattern: ^/(?!/)(.*)
- Path: https://webmail.domain.com
For more info please google for "apache redirect"
I am not sure about Lion but the redirection did not work automatically in Snow Leopard. So like I did in Snow Leopard I create two website:
1. https://example.com, select SSL certificate, port 443 (it change automatically if you select certificate first)
2. http://example.com, no certificate, port 80 and add redirection: For "When users visit:" choose "This website" and for "Redirect them to:" coose "example.com (SSL)"