IMP is the Internet Messaging Program (formerly, among other things, the IMAP webMail Program), a PHP-based webmail system and a component of the Horde project. IMP, once installed, accesses mail over IMAP thus requiring little to no special preparations on the server on which mail is stored.
About the Horde Project:
Are you a PHP application developer? The Horde Framework provides backend-independent interfaces for dealing with preferences, logfiles, MIME, hierarchical data, authentication, data formats, encryption, forms, session handling, file storage, remote procedure calls - and more is on the way.