[imp] smtphost in servers.php not overriding mailer in Horde's conf.php?

[Alain Fauconnet]

Hello List,

I've been pulling my hair out for a while on this one. My web mail is
supposed to route outgoing mail through another server and I found out
that it was actually connecting to SMTP on localhost to send mail out.

In IMP's servers.php I have:

$servers['webmail'] = array(
    'name' => 'webmail.ait.ac.th',
    'server' => 'webmail.ait.ac.th',
    'hordeauth' => false,
    'protocol' => 'imap/notls',
    'port' => 143,
    'folders' => 'INBOX.',
    'namespace' => '',
    'maildomain' => 'ait.ac.th',
    'smtphost' => 'smtp.ait.ac.th',
    'smtpport' => 25,
    'realm' => '',
    'preferred' => '',
    'dotfiles' => false,
    'hierarchies' => array()
);

...and the comment at top of the file says:

 * smtphost: If specified, and $conf['mailer']['type'] is set to 'smtp',
 *           IMP will use this host for outbound SMTP connections.  This
 *           value overrides any existing $conf['mailer']['params']['host']
 *           value at runtime.

In Horde's conf.php I just have:

$conf['mailer']['type'] = 'smtp';

...and no other $conf['mailer'] setting.

Still, IMP sends mail out through localhost.

If I force:
$conf['mailer']['params']['host'] = 'smtp.ait.ac.th';
$conf['mailer']['params']['port'] = 25;
$conf['mailer']['params']['auth'] = '0';
in Horde's conf.php, the problem goes away.

I'm probably overlooking something here, but I can't find it!
Would someone please hint me?
Horde-3.0.10, imp-h3-4.0.4 (I know, a bit old, but I can't upgrade yet)

Greets,
_Alain_