Comprobar en
/var/log/psa-horde/psa-horde.log
el error.
Si es:
Aug 05 11:22:22 HORDE [error] [imp] FAILED LOGIN 84.77.246.221 to localhost:143[imap/notls] as disvir@teatres.org [on line 258 of “/usr/share/psa-horde/imp/lib/Auth/imp.php”]
Ir a /etc/hosts
y añadir, sino está:
127.0.0.1 localhost localhost.localdomain
Info aportada por Javier Montalvo