Hi, Ethan.
Yes, I recognize the MX record (gator4143.hostgator.com). I reached out to my host provider (Hostagator) and made the MX record change as they recommended to me to test if it would work.
My host provider also determined that my email server IP address may have been blacklisted in a SORBS Spam database. I reached out to SORBS and requested delisting the email IP from their spam blacklist.
SORBS sent me an email informing me they have delisted the IP. I'm now waiting for the change to take effect/propagate across their DNS servers so we can test if the email non-delivery issue is now solved.
Apparently, it can take up to 48 hours for the outdated DNS information to be removed from DNS caches around the world.
Thanks.