Looks like this goes back to the days when Internet with TCP/IP was not yet the only option and when there was also other networks available (Chaosnet). [1]

For the A records you can different classes of addresses. Nowadays we are using the "IN", but the spec also supports others like "CH" (Chaos) and "HS" (Hesiod).[2]

MX record points to mail.example.org (A-record). mail.example.org could then point to two different addresses in the different networks, using the different classes. Depending on what network the client is connected to, it can request specify the class in the query and get address on the correct network.

I guess they could have baked this in to the MX records as well, but that would have made it more complicated.

Disclaimer: Just googled this up.

[1] https://serverfault.com/questions/663112/why-cant-mx-records... [2] https://serverfault.com/questions/220775/what-does-the-in-me...