HORDE [error] [imp] sendmail returned error code 65

[Harald G.]

Hallo,
i can log in to imap but i can't send an email with 'imp'.

Horde is on 192.168.6.6
qmail is on 192.168.6.5

my servers.php
$servers['_prompt'] = array(
    'name' => 'Choose a mail server:'
);

$servers['imap'] = array(
    'name' => 'IMAP COURIER',
    'server' => 'webmail.mymail.de',
    'protocol' => 'imap/notls',
    'port' => 143,
    'folders' => 'INBOX.',
    'namespace' => '',
    'maildomain' => 'example.com',
    'smtphost' => 'webmail.mymail.de',
    'realm' => 'example.com',
    'preferred' => ''


This is the ../horde.log when i send an email with 'imp'.

Jul 22 21:39:59 HORDE [error] [imp] sendmail returned error code 65 [on line
859 of "/usr/local/www/data-dist/horde/imp/compose.php"]

(I have also SquirrelMail installed on 192.168.6.5, it runs good)

Thanks in advance Harald