Issue Mail forwarding builds error message

[Thure Kjer]

Hi,
we have two mail adresses inhouse and use mail forwarding for both of them. When a client writes an email at my adress my colleague gets it also. That works fine until last week. Now we both still get the mails, the sender gets the following error message:

This is the mail system at host dieinfografen.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

<kjer@die-infografen.de>: mail forwarding loop for kjer@die-infografen.de
Reporting-MTA: dns; dieinfografen.de
X-Postfix-Queue-ID: AAF132007E
X-Postfix-Sender: rfc822; SRS0=SA8t=H4=me.com=sulaco.graphics@die-infografen.de
Arrival-Date: Wed, 9 May 2018 13:13:14 +0000 (UTC)

Final-Recipient: rfc822; kjer@die-infografen.de
Action: failed
Status: 5.4.6
Diagnostic-Code: X-Postfix; mail forwarding loop for kjer@die-infografen.de

How can we fix it?

Regards,

Thure Kjer