Apache2 URL Rewrite with Reverse Proxy

I haven't set up a https reverse proxy yet but I'm about to do it right now

as far as I can tell your proxy definition is in the wrong place.

Your RewriteRule redirects you away from your http virtual host to your https virtual host, so there's the place where the proxy config has to go.

EDIT: basically I meant this:

remove the proxy from the vhost on port 80

<VirtualHost *:80>
        ServerAdmin admin@mydomain.com
        ServerName mail.mydomain.com

        RewriteEngine   on
        RewriteCond     %{SERVER_PORT} ^80$
        RewriteRule     ^(.*)$ https://%{SERVER_NAME}$1 [L,R]



        ErrorLog /var/log/apache2/webmail_log
        CustomLog /var/log/apache2/webmail-access_log combined
</VirtualHost>

and add it to the vhost listening on port 443 (https)

<VirtualHost *:443>

... other vhost config ....

        SSLProxyEngine on

        <Proxy "*">
                Order allow,deny
                Allow from all
        </Proxy>

        ProxyPass / https://192.168.1.6/webmail/
        ProxyPassReverse / https://192.168.1.6/webmail/
</VirtualHost>

I haven't tested this exact setup but I think it should work like this...

You need to do this via two independent <VirtualHost *:X> directives. Your HTTP directives go inside <VirtualHost *:80> while your HTTPS directives go inside <VirtualHost *:443>. Adjust as required if your server has multiple Address Based or Name Based virtual hosts configured. See the Apache 2 documentation for full details.