Google Apps for Nonprofit is free for 501(c) nonprofits.

Here are a few of it's features,

• 25 Gigabytes of instantly accessible email storage (over 250,000 emails)
• You get to keep your current email address, myemail@mynonprofit.org
• Access your email safely and securely from anywhere on any computer; home, work, or on vacation
• Transfer all your current email into your Google Apps email account
• Access your email from the web or use any email program such as Microsoft Outlook
• Google Apps is more than email, it also features a calendar that can be accessed from any computer a web browser, Microsoft Outlook, smartphone, or tablet