I'm using courier-imap on my server. You will, however, need a custom template fragment.
/etc/e-smith/templates-custom/home/httpd/html/horde/imp/config/servers.php/120IMAPServers
Contents are:
$servers['imap'] = array(
'name' => 'IMAP Server',
'server' => 'localhost',
'protocol' => 'imap',
'port' => '143/notls',
'folders' => '',
'namespace' => 'INBOX.',
'maildomain' => '{ $DomainName }',
'smtphost' => 'mail.{ $DomainName }',
'realm' => '{ $DomainName }',
'preferred' => ''
);