There's a third gorilla in play in the email space - more organizations, some quite large are dumping their infrastructure heavy traditional email client/servers and migrating to Google Apps. Google Apps are enterprise ready, secure, and cost effective.
The Gmail client is ubiquitous, with usability, large mailbox size support, and great searchability. Using the software as a service (SaaS) model, you can cut down on administration and support costs by moving into cloud computing.
One of the big Google Apps partners, CloudSherpas, recently hosted an event called Leaving Lotus. Click on the link to get access to their takaway presentations, including a migration guide Migrating from Lotus Notes/Domino to Google Apps.
Here is an article from the Google Enterprise blog that focuses on migrating from Exchange to Google Apps. Choosing Google Apps over Microsoft Exchange.