Access denied (in reply to RCPT TO command)

Hi,

I can't get around one problem using postfix and kolab to send mail
through an SMTP-Auth relay (mail.arcor.de). It's working with arcor, if
I do it manually, but from kolab it isn't working.

I get the following error:

postfix/smtp[5290]: D8E814188C: to=<[email protected]>,
relay=mail.arcor.de[151.189.21.116]:25, delay=1.3,
delays=0.07/0.02/0.66/0.5, dsn=5.7.1, status=bounced (host
mail.arcor.de[151.189.21.116] said: 554 5.7.1 <[email protected]>:
Recipient address rejected: Access denied (in reply to RCPT TO command))

There is a thread about that in some mailing archive, it does not help.
So the scenario is like this:

Every user does login on my servers IMAP and wants to send mail through
the local postfix. Postfix has to connect with some user and password to
mail.arcor.de and queue the mail there. Sender (RCPT FROM) should be
[email protected].

I have fumbled with saslauth and created a file called sasl_passwd where
I stored the relayhost user:password, which was then mapped with postmap
and included in main.cf as
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

I have an empty transport-file at /etc/postfix/transport, but tried as well

## Internal delivery:
mydomain.com  :
.mydomain.com :
# External delivery
*      smtp:[mail.arcor.de]

which gave me an unknown transport error instead.

Well, what do do systematically next?

regards,
Henning