Choosing a mail server is never an easy task. Email management itself is a complex endeavor, and this is reflected in the wide variety of tools we choose to create, edit, send, organize and reply to the vast numbers of email messages that crisscross the globe on a daily basis.
Email management should be a reflection of the organization's email persona, in terms of how it conducts business with its customers and suppliers, and how its users interact with each other and with outside entities. Organizations with a large, mobile sales force will have vastly different needs than a small company with a local customer base. Each mail server product has its strengths and weaknesses, but it's never a mistake to start by identifying business needs first. This should trump seeking product features for their own sake.
You can greatly simplify email management by forgoing some of the more elaborate accoutrements (such as mobile mail, LDAP and ActiveSync) if you don't need them. If you do need these features, make sure the product has them, and ask for a hands-on demo to confirm. Most importantly, take the time to test.