We recently started to send out a higher amount of E-Mails (invitations to a private event) to a select number of people. Now from some of those people using office365 for Mails, we were informed that our mail landed in their spam folder, which is terrible for us, as others might not have seen their invitation in the spam folder.
I know there are a lot of guides with possible issues when it comes to spam filters, and I have been following them, without any satisfying results yet, but I think I found a strange detail and don't quite know what to do with it.
Using spam testing tools (mostly litmus https://litmus.com/) also showed it being labeled as spam by the Microsoft Exchange Online Protection. However, when I tried running the same test, sending the same message from the same SMTP server only originating from a different computer, there was no problem. I re-tried both tests with the same results.
Now to me, it looks like the mail was filtered based on the originating computer. Is this a thing that might be happening and if so, what could I do, or am I rather missing something else?
Some more details: The E-Mails are always sent from a custom Django web app connecting to a remote SMTP server. From the production Server on our VPS, it was labeled spam, whereas from my dev server running on my local machine there seemed to be no problem. The respective spam confidence levels were 5 and 1, and according to the headers the message was filtered by content
Here are the headers for the filtered Message: https://pastebin.com/F1kKZjnK