Gmail has detected that this message 550-5.7.1: how to fix sending mail with azure communication

Question

I'm migrating my site from AWS to Azure.

I migrated the VM containing a django site.

I have a newsletter with a form to subscribe.

I set up azure communications by using MX record, spf, dkim and DMARC.

Now I got on azure communication logs the follow

Gmail has detected that this message 550-5.7.1 is likely suspicious due to the very low reputation of the sending 550-5.7.1 domain. To best protect our users from spam, the message has been 550-5.7.1 blocked. For more information, go to 550 5.7.1 https://support.google.c'

With AWS ses didn't have problems (actually I didn't had a CAPTCHA on the form so I was sending lots of email!)

How can I overcome this problem?