wrong E-Mail Sender Adress from ASL

Hello!

I have some problems with the sender address of "Mailer Daemon" of ASL.
It will always send to anonymous@xx.yyyyyy.de.
As this domain doesn't exist, I still receive following error message:

"550 Sender domain not found in DNS."

Does anybody know where I can adjust or even correct the sender address?

Thanks 

Stefan

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