horde Auth_imp: Required IMAP extension not found.

I installed the horde webmail groupware on my rackmounted server as it is described in the horde webmail wiki for debian. After I tried to login to my mailaccount via horde I received the following error message

A fatal error has occurred
Auth_imp: Required IMAP extension not found.
Details have been logged for the administrator.

The solution was to install the imap extension for php5:

apt-get install php5-imap