That’s not a URL that you get get. It’s only for POST.
And if you did Straight-forward incoming mail, you don’t have a POP3 host. And the MX for discourse-staging.lmera.ericsson.se doesn’t resolve to your Discourse server, so the error message you’re getting on mail delivery isn’t from your Discourse server.
With that many problems, I don’t think that you’ll be able to get it sorted out here.
If you have a budget, my contact information is in my profile.