Amazon SES - Scale your business email & marketing messages with Amazon Simple Email Service (SES), a cost-effective and reliable cloud email service. Sign up today.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Mailgun - A set of powerful APIs that enable you to send, receive and track email from your app effortlessly whether you use Python, Ruby, PHP, C#, Node.js or Java.
Roundcube - Web-based IMAP email client
Mandrill - Mandrill is a new way for apps to send transactional email. It runs on the delivery infrastructure that powers MailChimp.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.