Ideactif-en.mail.protection.outlook.com - Welcome emails doesn't arrive in our users' inboxes under outlook

We use Auth0 to manage our users.

When verifying email addresses during signup, Auth0 email for email address verification is not delivered to our users who use outlook.

We use Mailgun for sending emails.

In the Mailgun logs we can see that the email is delivered but then blocked by
“mx-host”: “ideactif-en.mail.protection.outlook.com”. (and therefore is not even present in the spam mailbox of our users)

We have checked the spam scores and blacklists; neither our email, nor our domain, nor the shared IP used by Mailgun is a concern.

Could you please tell me how to correct this?

It may be complex to review this situation by only knowing the outcome; outlook blocking them. Having said that if you haven’t done so already you should try to analyze a full raw verification email message through (https://mxtoolbox.com/EmailHeaders.aspx) and see if that hints at the possible problem.