Topicbox - Put team emails into one centralized, searchable place
HEY - Email at its best, new from Basecamp.
Nova Inbox - Personalize emails in seconds, right from your inbox
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Front - The platform for exceptional customer service at scale.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.