[imp] e-mails sent via IMP have strange header

[James Brown]

Dear All,

I have a small issue when sending mail from IMP (IMP 3.2, php4, 
apache1.3), which is causing me some serious head scratching. The 
problem is that the headers of mails sent from IMP always contain:

X-Authentication-warning: host.mydomain.com: www set sender to
me at mydomain.com using -f

I do have sendmail running locally, but I can't see where this '-f' flag 
is being set? Could anyone enlighten me please? (relevant config files 
shown below)

Any ideas would be much appreciated.

TIA,

James.

**/horde/config/horde.php**
[...]
$conf['mailer']['type'] = 'sendmail';
$conf['mailer']['params'] = array();
$conf['mailer']['params'] = array(
     'sendmail_path' => '/usr/sbin/sendmail',
     'sendmail_args' => '-oi'
);
[...]

**/usr/local/etc/php.ini**
[...]
; For Unix only.  You may supply arguments as well
;(default: "sendmail -t -i").
; staying with the default as this is commented out
;sendmail_path =
[...]

**/etc/mail/freebsd.cf**
[...]
FEATURE(`masquerade_envelope')dnl
MASQUERADE_AS(`mydomain.com')
[...]