SquirrelMail is a standards-based webmail package written in PHP4.  It
includes built-in pure PHP support for the IMAP and SMTP protocols, and all
pages render in pure HTML 4.0 (with no Javascript) for maximum compatibility
across browsers.  It has very few requirements and is very easy to configure
and install.  SquirrelMail has a all the functionality you would want from an
email client, including strong MIME support, address books, and folder
manipulation.

Homepage:
https://www.squirrelmail.org/
