Sorry, but I can not find a propre information in this document:
for "The 'vinfo' hook has been removed.  Use Horde's 'authusername' 
and/or IMP's
'preauthenticate' hooks instead."
I tried to play with this in Horde/config/hooks.php but I did not succeed

Regarding
"All hooks that are specific to IMP have been moved from the
``horde/config/hooks.php`` file. Move your existing IMP Hooks from there to
``imp/config/hooks.php``."
this was already in imp/config/hooks.php in Horde3/IMP4. Should I 
copy/paste the code to Horde5/IMP4 ?

Thanks for your support,
Antoine.

See imp/docs/UPGRADING

In my Horde3/IMP4, I used to have a custom hook function in the 
horde/imp/config/hooks.php to support multidomains that I am hosting.
The user points its browser to mail.domain.com, the the login window 
displays "@domain.com" at the end of the username field and horde/imp 
provides username@domain.com to the IMAP server 
(qmail/vpopmail/courier-imap).

This function looks like this:
if (!function_exists('_imp_hook_vinfo')) {
     function _imp_hook_vinfo($type = 'username')
     {
         $vdomain = getenv('HTTP_HOST');
         $vdomain = preg_replace('|^(web)?mail\.|i', '', $vdomain);
         $vdomain = preg_replace('|^imp\.|i', '', $vdomain);
         $vdomain = String::lower($vdomain);

         if ($type == 'username') {
                 if ($vdomain == '') {
                         return $_SESSION['imp']['user'];
                 } else {
             return (preg_replace('|@.*$|', '', 
$_SESSION['imp']['user']) . '@' . $vdomain);
           }
         } elseif ($type == 'vdomain') {
             return $vdomain;
         } else {
             return PEAR::raiseError('invalid type: ' . $type);
         }
     }
}

I did not find such a feature in Horde4/IMP5.
I have tried to play a bit with the hook public function 
authusername($userId, $toHorde) in horde/config/hooks.php
but users still need to provide user@domain.com to logon (and they are 
recognized as user@domain.com@domain.com by horde) and the logon 
window does not displays "@domain.com" after the username field.

Am I missing something ?

Thanks for your support,
Antoine.