The library presents a fully abstracted interface to an IMAP/POP3 server, freeing a developer from worrying about the complex underlying protocols.
The library provides a native-PHP driver that does not require additional, optional extensions to be built into PHP at compile time.
A full list of features can be found on the Features page.
Why not use the PHP IMAP extension?
Horde/Imap_Client is significantly faster, more feature-rich, and extensible when compared to PHP's imap (c-client) extension.
Don't be confused: almost every so-called "PHP IMAP Library" out there is nothing more than a thin-wrapper around the imap extension, so NONE of these libraries can fix the basic limitations of that extension.
Some history: development of Horde/Imap_Client was primarily funded by a large Internet Service Provider precisely because the imap extension could not support the volume of traffic present on their systems. Horde/Imap_Client has been powering their web-accessible mail backends for more than four years, supporting thousands of concurrent connections and millions of active accounts.
Horde/Imap_Client is used by the IMP Webmail Application, the most advanced open source webmail program available.
Additionally, the Horde Project uses the Horde/Imap_Client library to synchronize a user's e-mail account with ActiveSync devices.
However, the Horde application DOES NOT need to be installed on your server to make use of Horde/Imap_Client: installation will automatically install all required dependencies.