Microsoft Exchange vs Open-Source Alternatives

Hi everyone, I am just wondering which one is more preferable among network administrators who manage their own email system. Previously, my company have outsourced email solution, which I dislike a lot. Setting aside the concern of budget since we’ve received a free copy of Exchange 2003 Standard from Microsoft for being a non-profit. Anyhow, in terms of reliability, usefulness, support, and speed, Exchange or other Opne-Source alternatives such as Zimbra will be more favorable? So I decide to turn to experienced adminsitrators. What do you think?