Courier is described as 'was the evolution of the Calypso email client' and is an app in the Office & Productivity category. There are more than 10 alternatives to Courier for a variety of platforms, including Windows, Linux, Mac, BSD and Android. The best alternative is Thunderbird, which is both free and Open Source. Other great apps like Courier are Microsoft Office Outlook (Paid), Microsoft Mail and Calendar (Free), Claws Mail (Free, Open Source) and Postbox (Paid).
Claws Mail is an email client (and news reader), based on GTK+, featuring Quick response Graceful, and sophisticated interface Easy configuration, intuitive operation Abundant features Extensibility Robustness and stability
Postbox is an email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even the most demanding workloads.
Kontact is the integrated Personal Information Manager of KDE. Kontact suite unites mature and proven KDE PIM applications under one roof. Thanks to the powerful KPart technology, existing applications are seamlessly integrated into one.
A fully fledged, free email server solution, an open source project (GPL v2). Use official binary packages from Linux/BSD distributions, with both i386 and x86_64 support. Easy, fast deployment in LESS THAN 1 MINUTE. View YouTube Video to check how easy it is.