Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client (mailer, MUA).
* Simple, beautiful, and well-polished user interface
* Comfortable operationality which is built in detail
* Well-organized, easy-to-understand configuration
* Lightweight operation
* High reliability
* Security features (GnuPG, SSL/TLSv1)
* Powerful filtering and search
* Junk mail control
And then some. See website for details.
Similar applications:
Outlook Express, Becky