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From my initial read, seems like SMTP relay is the way to go with a device like this

I was under the impression that you just had to have an IP with an A record that matched the reverse DNS, even if it's not for the A record that's your email domain?

It depends on the MX you're delivering mail to. Some aren't restrictive at all, some will reject if there's an A/PTR mismatch, etc. I make use of FCrDNS on my mail systems.

What is "valid"? I've been delivering personal email for 2 years from an IP whose reverse is like 1-2-3-4-static.hfc.comcastbusiness.net, and that name does not match what I put in my MX or HELO.

Now, granted, there are probably some systems that have issues with this, and maybe I do have failed deliveries I didn't notice, but you did say no email will deliver at all.

Valid is generally the one that matches your HELO, and in a perfect world your machine's hostname.

The mail systems that I run (at/for an ISP) would block your mail (although I am probably more strict about what I will accept than most).