[horde] Problem with sending invitations via kronolith

[Jens Grüntjes]

Hello list,

I'm using the latest pear versions of Horde Groupware 5.1.2.

I recently configured Horde globally to deliver mails via port 587 (as  
proposed lately on the list) like this:

$conf['mailer']['params']['host'] = 'localhost';
$conf['mailer']['params']['port'] = 587;
$conf['mailer']['params']['localhost'] = 'localhost';
$conf['mailer']['params']['auth'] = true;
$conf['mailer']['type'] = 'smtp';

Sending mails from imp works perfectly but sending invitations by  
kronolith does not. This is the log entry from horde:

2013-09-12T10:02:54+02:00 DEBUG: HORDE [kronolith] Beim Verschicken  
einer Terminbenachrichtigung an xx.xx at domain.de ist ein Fehler  
aufgetreten: Server denied authentication. [pid 8225 on line 27 of  
"/usr/share/php/Horde/Core/Notification/Handler/Decorator/Hordelog.php"]

My postfix log only shows:

Sep 12 11:23:16 xxxx postfix/smtpd[7722]: connect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: disconnect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: connect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: disconnect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: connect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: disconnect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: connect from localhost[::1]
Sep 12 11:23:16 xxxx postfix/smtpd[7722]: disconnect from localhost[::1]

This worked when I delivered the mails via port 25. Can someone advise me?

Thanks for your help
Jens