What is the difference between Professional and Corporate level subscriptions?

Corporate is everything in Professional plus the ability to store batches (lists) of addresses and test them automatically on a schedule.

Professional is access to the raw tools, Corporate makes them easy to use over and over.

Professional tests things one time, Corporate is "set it and forget it" continuous monitoring of one or more email systems.

You can achieve HIPAA Compliance with Corporate. See email compliance

Corporate also has Internet Packet Sniffer and Protocol Analyzer -- very powerful tools that take some time to learn and setup.