Moving on to bigger and greater things! Exchange 2007!
Exchange 2007 introduced new server roles (or at least renamed them), Hub Transport Server, CAS server, and Mailbox Server. It also came with other server roles like Edge and Unified but we won't get into those for now...cause I dont know squat about them.
Exchange 2007 also introduced the new name for the default OWA virtual directory /OWA. No more /Exchange from here on out.
This brings me to the point of this post. Normally given the right configuration your 2007 server should be able to process a request of mail.domain.com/exchange and if a 2003 user logs in it will proxy that connection back to the 2003 server.
Except sometimes it doesn't. In one very specific case redirection will fail. If your 2007 server has the Mailbox role installed onto the same server as the CAS role then you are out of luck, better go buy another server.
The worst part about all of this is that I only found this documented in one tiny spot buried in a Technet article. I felt like an archeologist discovering the struggles of an ancient civilization.
Article in question
Next time you have a client that is upgrading from 2003 to 2007 in the year 2016, watch out for this gotcha...