[BlueOnyx:05600] Alias not working

[Klein Joachim]

  Hy to the List!

I have a new Site called mail.customer.at and customer.at as Email-Aliase.
The User is called zmk_office with office as emailalias.
mx-Record: customer.at in MX mx1.mydomain.at
A-Record: mx1.mydomain.at in A IP of the Server
Cname: mail.customer.at IN CNAME mx1.mydomain.at

If I send a mail to zmk_office then the mail is in the mailbox, but if I
send a mail to the Alias "office" then the mail is returned with the error
<office at customer.at>... User unknown

I´m using the Solarspeed-Special-Appliance with an installed 
Spamfilter-Package.

On the other websites this working, only on this website it sucks

Thanks
Klein Joachim