Plesk gives following error for horde

Auth_imp: Required IMAP extension not found.
It requires IMAP module of php to be installed on the server.

To install the php-imap do the following

1) download and install repository

wget -q -O - http://www.atomicorp.com/installers/atomic | sh
2) check the version of php

If php has latest version then install the imap with following command

yum install php-imap
If php has older versions then compatible modules is to be installed. Under this situation
following command can be used to install imap.

yum install php-imap-5.1.6
(for php version 5.1.6)

3) restart httpd

/etc/init.d/httpd
restart

4) restart plesk

/etc/init.d/psa
restart.