Well, as the saying goes, it’s always DNS.
There’s not much chance on getting it figured out with it using placeholders since neither you nor whoever is helping you can check anything, but I guess here’s an attempt anyway.
The MX record should be
discourse.example.com pointing to discourse.example.com with any priority.
Don’t change anything about the TLD’s email. Just the MX record for the mail receiver, which is the same hostname as Discourse.