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 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,
...Licence : GPL 
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