I am running CentOS and PLesk 9.0.1. All the mail I am trying to send from Drupal-based and WordPress-based sites is getting stuck in the mail queue which I can view through PLesk. Also, when I try to access the Horde Webmail via Plesk I get an error saying "imp/login.php was not found on this server"

I tried the edits suggested here to no avail:

I realized that there is in fact no login.php file present in the horde/imp directory on the server. This is likely one of the problems but I have no idea what is going on.