After upgrade from RC3 to RC4 problem with postfix

[DaSilva]

I have a problem with postfix after an upgrade from RC3 to RC4.
I have an e-mail address which can only send e-mails but cannot receive e-mails.
The login in the webmail client works and I can send e-mails from that address from webmail and Outlook.
But if I send an e-mail to that address I only get back a "Undelivered Mail Returned to Sender" mail with this content:

Code:

This is the mail system at host domain.de.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

                   The mail system

<support@domain.de>: unknown user: "support"

details.txt:

Code:

Reporting-MTA: dns; domain.de
X-Postfix-Queue-ID: F3C3A1C10003
X-Postfix-Sender: rfc822; test@domain2.de
Arrival-Date: Sat, 29 Mar 2008 18:22:43 +0100 (CET)

Final-Recipient: rfc822; support@domain.de
Original-Recipient: rfc822;support@domain.de
Action: failed
Status: 5.1.1
Diagnostic-Code: X-Postfix; unknown user: "support"

How can I fix this?
Thanks for help!