[scintill76]

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.

[jlgaddis]

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).

[blfr]

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