Custom Return Path | Permanent failure :invalid-sender

Custom Return Path | Permanent failure :invalid-sender

Dear community,

I configured the customer return path by adding the CNAME for "zcmp" in my domain (CloudFlare, proxy mode disabled). The campains UI marked the configuration as valid/checked and I moved on to prepare an email campain.

From the last campain I however noticed many bounces with the following message: Permanent failure :invalid-sender 

Action: failed
Status: 553
Diagnostic-Code: 553 #5.1.8 Domain of sender address <campaign_xyz@zcmp.xyz> does not exist 
X-ZCMTA-BounceCategory: invalid-sender

Besides that many not received the email, the contacts been auto added to the bounce list and presumable been removed from future mailings....

What to do?