NaSMail is a standards-based webmail package. It includes support for the IMAP and SMTP protocols, and all pages render in HTML 4.01 with minimal JavaScript and CSS. It has very few requirements and is very easy to configure and install. NaSMail has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation. NaSMail is based on SquirrelMail 1.4.x. It includes internationalization and other code changes from SquirrelMail version 1.5.x.
