SquirrelMail is a standards-based Webmail package.
It includes built-in pure PHP support for the IMAP
and SMTP protocols, and all pages are rendered in
pure HTML 4.0 for maximum compatibility across
browsers. It has very few requirements, and is
very easy to configure and install. It has all the
functionality you would want from an email client,
including strong MIME support, address books, and
folder manipulation.