Lots of email hosts are like that, btw. There was a time, when the Internet didn't look much like it does today - especially when UUCP reigned supreme - when n

Lots of email hosts are like that, btw. There was a time, when the Internet didn't look much like it does today - especially when UUCP reigned supreme - when nearly all email domains did not have associated IP addresses. Local ISP's that give custom email addresses to their users also nearly always do the same thing. If you want to go down a rabbit hole, look up how to do bang path email routing. It still works today, even if nobody does it. The only part that matters is that the sending mail server can find an MX record for the receiving domain's mail server. There is no requirement for the named domain to have its own IP address as long as whatever domain the MX points to knows where to send the incoming mail.