[horde] Horde/IMP domains

[Antonio]

Hello.

I have this horde/imp/config/servers.php
...
$servers['cyrus'] = array(
    'name' => 'Cyrus IMAP Server',
    'server' => 'localhost',
    'hordeauth' => false,
    'protocol' => 'imap/notls',
    'port' => 143,
    'folders' => '',
    'namespace' => 'INBOX.',
    'maildomain' => 'mydomain.org',
    'smtphost' => 'localhost',
    'smtpport' => 25,
    'realm' => '',
    'preferred' => '',
    'admin' => array(
....

Why when I compose a message, my identity is user at example.com and not 
@mydomain.org?

Thnaks!