But if I can’t define a new email address (like x@mydomain.com), then how can I ever complete this?
I can use a non-domain email address, but Discourse will only send transactional emails from domain-based addresses (if my understanding of the situation is correct).
For clarity: I have indeed specified a domain-based email address as the sender (within Discourse, and specifically within the app.yml file or whatever it is), but [1] I have not added this as a Sender Address in Mailjet because my understanding is Sender Addresses are for any addresses that do NOT share the same domain, and [2] I cannot create a domain-based email address because I am not running a mail server (the entire point of Mailjet, as I understand it at least, is to avoid having to set up and run your own mail server, which would be extremely unreliable for deliverability).