Emails from our custom domain ends up in quarantine

Hello, I have an issue with emails coming from one of our custom domains being detected as phishing for identity theft reason.

We've got the main domain on Microsoft 365, let's call it: "domain-example.com" and the second domain on IONOS : "domainexample.com" When we receive emails from this second domain it is detected as phishing since it is a domain name very close to our main domain, therefore those emails are placed into quarantine.

How do I resolve this issue? Can I indicate Microsoft 365 that this second domain is owned by our company, therefore the emails coming from this domain aren't identity theft attempts?

I found how to add a domain on Microsoft 365 Admin Center but I don't know what are the consequences of this action, I want to keep my domain on IONOS and send emails from their web server.

All I want is to stop emails from our company to end up into quarantine or spam folder.

Thank you.