Yes,

You have to add a MX record to the DNS for “mydomain.org”. The MX record points to “mail.mydomain.org”. When mail servers resolve “user@mydomain.org” they will query the MX record for “mydomain.org” and get told to fetch email from “mail.mydomain.org”. That DNS can point to any IP you want to use as your email server.

Yes.

I like that kind of QA. It’s so clear and Expressive and understandable :)