Issue : Two SMTP domains, one single Exchange Server!?

I am supposed to work on a new Microsoft Exchange scenario. A customer (abc.com) has acquired another company (def.com) and they want a single exchange server handle both smtp domains and I haven't done this before! Once I worked on a scenario where the customer wanted to share the same smtp domain with another exchange server! They needed more space at their office so moved some of their departments to a different location and then two exchange servers with the same smtp domain were in place. The following link will help to accomplish this:

How to Configure Exchange to Forward Messages to a Foreign Messaging System That Shares the Same SMTP Domain Name Space

This new one doesn't seem difficult but a couple of things must be taken into account :
1- The customer is using a scheduled dial-up connection for mail delivery
2- The exchange server is using another mail server on the public network as its smart host.
3- A third party pop3 connector is handling delivery to the smart host
I think The difficult part is calling the ISP and asking for MX records!