It's been a while since I worked with Courier (I still prefer it to WU, which is what's included w/ SME, but it was too much of a pain to keep changing it after every update), but I can think of at least one change you'll need to make. Edit /etc/e-smith/templates/home/httpd/html/horde/imp/config/servers.php/40Servers to read as follows:
$servers['imap'] = array(
'name' => 'IMAP Server',
'server' => 'localhost',
'protocol' => 'imap',
'port' => 143,
'folders' => '', // this is two single quotes, changed from Mail/
'namespace' => 'INBOX.', // be sure to get the period after INBOX
'maildomain' => '{ $DomainName }',
'smtphost' => 'mail.{ $DomainName }',
'realm' => '{ $DomainName }',
'preferred' => ''
);