Chuck Hagenbuch published a first version of IMP on Freshmeat in 1998. A constant stream of feature requests not all fitting for a webmail application led to the development of a more generic web application backbone: the Horde framework. The release of IMP 3.0 and Horde 2.0 was the first one with two truly separate components. Since then any deployment of IMP can only run on top of a Horde installation.
SAPO uses IMP to provide several million users with a webmail platform. The company also sponsored the initial development of the dynamic AJAX variant of IMP which was released as a separate package - named DIMP - with the release of Horde 3.2.
Support for mobile clients was provided within 2006 for the first time. At that time a WAP based mobile view was provided by a separate package called MIMP.