Once you have verified a mail account's username and password are correct and you are still getting the following error:

"Login failed."

You can take the following steps to try and correct the issue:

chmod u+w /home/httpd/vhosts/webmail/horde/imp/config/servers.php
chmod u+w /home/httpd/vhosts/webmail/horde/config/horde.php

Around line 95 of /home/httpd/vhosts/webmail/horde/imp/config/servers.php change:

'server' => 'localhost',

To:

'server' => '<main.server.ip.address>',

Also change line 236 of /home/httpd/vhosts/webmail/horde/config/horde.php from:

$conf['mailer']['params'] = array('host' => 'localhost');

To:

$conf['mailer']['params'] = array('host' => '127.0.0.1');