Hello Sten, just to add to what Sean said about the issue you are experiencing I found this article that is related to the issue you are experiencing based on the error: Remote Server returned '550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7555). We have this article related to that error https://support.zendesk.com/hc/en-us/articles/4408822468506. You may use this as reference for your domain admin or provider to work on the error you are getting.

All the best,