Zimbra is an Ajax Microsoft exchange competitor with a webmail service that thousands of businesses and organizations use to handle email, contacts and calendaring. They also offer a great mobile solution. The core product is open source, and Zimbra has a higher end version that sells for $25 per person per year (with various discounts).
Since their launch they’ve grown. And grown. They had 4 million “paid mailboxes” In October 2006. Next Monday they will announce that they now have more than 6 million paid mailboxes over 1,300 customers, a growth of 50% in three months. Sixty percent of their customers are being serviced through resellers.