Outbound Glossary
MX record
The DNS entry that says where your domain receives mail. A sending domain with no MX record looks fake, because real mailboxes can be replied to.
An MX record points at the server that accepts incoming mail for a domain. It has nothing to do with sending, which is why it gets forgotten on domains bought purely to send campaigns from.
That omission is a signal. A domain that sends but cannot receive is behaving like something no human sits behind, and receivers notice. If you set up a sending domain, give it a working inbox, and read it. The replies you get there are also the fastest feedback loop you have on whether the campaign is landing.