Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.
HEY - Email at its best, new from Basecamp.
FastMail - Email, calendars, and contacts to highest standards of security, privacy and reliability.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Rackspace Email Hosting - Rackspace Email Hosting is a platform that provides business with web-hosted email services.
InstAddr - Disposable email address application.