Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
HEY - Email at its best, new from Basecamp.
Soverin - Soverin is the honest email service that doesn’t sell your data.
InstAddr - Disposable email address application.
NeoMail - NeoMail is a web-based mail client that allows users to access their mail spools and send messages.