Problem sending emails in Apex using email delivery in an autonomous database

I followed the guide on how to use the email delivery service in Apex, but I get an error with the count=0 error in my test environment. I followed all the steps:

• Create a user for SMTP credentials
• Add it to a group and assign email sending policies
• Configure SMTP credentials in Apex
• Add it to the approved senders list
  I have a dev environment where the exact same thing has been done, but it can still send emails.

And when I run the script, I get the following error:

BEGIN
APEX_INSTANCE_ADMIN.VALIDATE_EMAIL_CONFIG;
END;
/