I have just added a secondary domain to my workspace and now I cannot receive anymore mails

[RenePf]

Hello, I have integrated a domain of mine via the Google Workspace Aliases.

Following these steps: https://knowledge.workspace.google.com/kb/how-to-add-a-domain-alias-000007733?hl=de

I am using Cloudflare and everything was set up by google automatically. Now, I tried to add my email to the mail client with extra accounts under the mail config of my workspace user. Google asked to send a verification mail. 550 5.1.1

However, now I am getting a 550. 5.1.1. error mail, that the mail either does not exist or is not configured. WTF? I have checked my MX records. And according to these, everything is fine. Google Set the MX Records automatically.

Now I have reverted the changes, but currently google does not seem to be able to fetch mails from my mail server. 

Either way, if I add the Google MX records, my Mail ceases to exist. If I add my domain registrar MX records, google cannot receive my mails.