When creating Calendar Events via the Graph API, the events are created correctly in the calendar of the organizer but the invites to the participants get bounced. The organizer gets a 'Delivery has failed...' message with the following error message:

```
Remote Server returned '550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653 AS(7171)

```

Sending emails manually through Outlook (web) from the organizer to the participants works fine.

The issue is intermittent. After changing the DNS server, everything worked fine for some time.

I checked <https://protection.office.com/restrictedusers> as suggested in the answers to similar posts, but this list is empty.

I also tried delisting the IP address (my own?) through <https://sender.office.com/>, but to no avail.

The tenant is linked to the Microsoft Developer Program and would allegedly have a 'bad reputation'. However, I don't understand how that would explain the fact that it sometimes works and sometimes doesn't when sending exactly the same API request and hence 'triggering' exactly the same emails...