I'm thinking that there's absolutely no reason or justification for this, but before I open my mouth and get some admin in deep kimchi, I thought I should ask.
Is there any circumstance in which an MX record should point to a loopback address? To me, this says any mail server that tries to send to this domain will instead send to themselves, and fail, but I'm not a mail guru, so maybe I'm missing something.
I came across the below when troubleshooting a why-aren't-we-getting-email?? issue for a client, and I'm having trouble wrapping my head around it. Maybe I'm overthinking things, though.
Best Answer
Short answer: It shouldn't.
Long answer: If the domain in question (DIQ) should not receive email, then putting a loopback address in for the MX record causes the sending server to attempt to connect to itself. This saves the DIQ a few measley bytes and possibly cleans up the firewall logs (if anyone is even watching) when other mail servers attempt to connect. However, in my opinion, the bandwidth savings is not enough to justify violating RFC 3330.