Roundcube webmail is a browser-based multilingual IMAP client with an
application-like user interface. It provides full functionality you
expect from an email client. It is fast and only needs a basic
php capable server and sqlite, mysql or postgresql.
Use roundcubemail-apache.conf from the doc directory as starting point.

Features:
    Drag-&-drop message management
    Full support for MIME and HTML messages
    Multiple sender identities
    Full featured address book with groups and LDAP connectors
    Find-as-you-type address book integration
    Threaded message listing
    IDNA and SMTPUTF8 support
    Spell checking
    Responsive skin (multi-device support)
    Shared/global IMAP folders
    Support for access control lists (ACL)
    Built-in caching for fast mailbox access
    Unlimited users and messages
    Import/export functions
    Plug-in API for flexible extensions
    XSS attack protection
    Support for PGP encryption