Incoming and Outgoing mail using astaro and exchange2010

Hello,

we have following situation:
We have a domain (example.com) at our ISP. At the ISP we also have email-mailboxes. In our LAN there is an exchange2010 server with PoCon installed. PopCon receives mails from the external mailboxes (ISP) and delivers those mails to the internal exchange-mailboxes.

The MX-Records of the domain example.com goes to the mailservers of the ISP.

What we would like to do is to route all incoming and outgoing mail from our exchange server through the astaro. Additionally we would like to access the ActiveSync feature from the internet.
We have a public ip-adress 123.123.123.123 (example)

How do we have to setup the astaro for this functionality.
I`m thinking about following:

1. Create subdomain mail.example.com at ISP
A-Record: 123.123.123.123
MX-Record: mail.example.com

2. Change records for domain example.com
MX-Record: mail.example.com

3. Configure Virtual Webserver in "Web Application Security"
mail.example.com -> 123.123.123.123 -> "internal exchange server [Port443]" for Exchange-Active-Sync

What do we have to do to get Email work incoming and outgoing?

I`m looking forward to some replys.

Thanks a lot!