SquirrelMail : SquirrelMail is a standards-based webmail package written in PHP.
SquirrelMail

SquirrelMail Screenshot
Overview
SquirrelMail is a standards-based webmail package written in PHP. 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 required) for maximum compatibility across browsers. It has very few requirements and is very easy to configure and install. SquirrelMail has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation.

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Features

  • Spell checking (squirrelspell)
  • Mail filters (filters)
  • Web-based administration of SquirrelMail (administrator)
  • A calendar (calendar)
  • An interface to submit bug reports semi-automatically (bug_report)

Examples of functionalities added by various third-party plugins include: