Hastymail2 is a full featured IMAP/SMTP client written in PHP.

Features:
  * Compatible with PDAs, phones, text browsers, and all mainstream browsers
  * W3C valid UTF-8 XHTML pages that can be configured to be delivered as type
    application/xhtml-xml for supported browsers
  * Multilevel theme system using CSS, PHP templates, and icon sets
  * The ability to use a database for storing user settings, contacts, and
    plugin data, with support for both Postgresql and Mysql
  * Flexible and powerful plugin system with documentation, examples, and many
    plugins like a calendar, auto complete, sound notifications, HTML mail
    composition, and more
  * Tunable smart cache to balance IMAP activity and PHP session size to
    maximize performance for different configurations
  * AJAX updating using a lightweight and simple system
  * Small page sizes to minimize load time and bandwidth use. Average page
    sizes including javascript with AJAX updating, CSS, and images is less
    than 100K without compression or caching, 50K when using gzip compression,
    and less than 20K when using gzip and normal browser caching
  * Multiple interface languages and enhanced message character set handling
  * Custom built RFC compliant IMAP class that handles all IMAP communications
  * No special PHP requirements, compatible with PHP4 and PHP5
  * Advanced IMAP search capabilities
  * Vcard based contact management including import/export ability
  * IMAP SSL and STARTTLS* support
  * Flexible sessions including support for cookie-less sessions and random
    session ids*
  * Tons more!

WWW: http://www.hastymail.org/