According to InfoWorld, Microsoft Exchange 2010 is the best mail server! :)
"Microsoft Exchange 2010 continues Redmond's longstanding market leadership in mail servers and related software. This release shows why the company's once-reviled product continues to be deployed widely in enterprises: it works, it scales, and it continues to gain important new features. In our tests, we saw improved management, administration, and reliability features plus an enhanced experience for users that includes automatic transcription of voicemails, delivery reports for messages, and extensive self-service options. Although light on wholesale changes and heavy on refinements, Exchange 2010 is nevertheless a major step up from Exchange 2007."
Source: InfoWorld's 2011 Technology of the Year Award winners