If you received an error when you open Horde email client :

A fatal error has occurred
Session cookies will not work without a FQDN and with a non-empty cookie domain. Either use a fully qualified domain name like “http://www.example.com” instead of “http://example” only, or set the cookie domain in the Horde configuration to an empty value, or enable non-cookie (url-based) sessions in the Horde configuration.
Details have been logged for the administrator.

In the file :
/usr/local/cpanel/base/horde/config/conf.php

Change the value for $conf[‘session’][‘usr_only_cookies’] = false