Email Notifications from Outlook integrated with Zoom and Calendly Blocked for Gmail Users – Error 550 5.7.520

Hi!

I use Outlook Calendar integrated with Zoom and Calendly to manage appointments. However, email notifications of appointments are not being delivered to Gmail users. The following error appears:

Remote server returned '550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)'

Invites sent directly from Outlook Calendar to Gmail users work without any issues, so the problem seems specific to the email notifications generated via the integrations. I’ve tested both Calendly and Zoom independently, and they are not the source of the issue.

How can I resolve this and ensure email notifications are delivered to Gmail users?

Thank you in advance for your assistance!