Hello,
This is a big decision that you are making!
What features would you like that you already have?
What features would you like that don't exist in Exchange 5.5?
Create both a Requirements List and a Wish List to help get started with that thinking.
Do you have any concerns about redundancy?
What is the average size of the email accounts?
What are you doing now for reputation and spam filtering at the network perimeter, mail server, and user level?
Does the upgrade need to be a Microsoft solution?
What else do you or your team have experience supporting?
Are you open to outsourcing any of the functionality if needed?
Do you have a budget?
Hopefully there are enough questions here to help you get started.
to Email Questions!