The setup: WHM/CPanel CentOS 5 server running Exim and Courier for mail services, and BIND for domain name services.
I recently moved servers. The old server was running a HIGHLY similar configuration, and all accounts were ported via WHM. However, the server is unable to send, and sometimes receive email.
Errors I am seeing (when I do get an error mail back) state:
450 4.1.8 : Sender address rejected: Domain not found
Edit for clarity: this is the error response from remote mail servers. Numerous independent mail servers come back with the same error.
(Email address is merely one valid example)
My first instinct of course was to check the domain records. However, k-t.org appears to have a valid record (including an MX record), even after running it through domain checks on a completely different server elsewhere and online. Note that the issue appears to happen with all the domains hosted on the server, not just k-t.org
I have also ensured that a PTR was created.
My Googling has only lead me to people who had fairly basic DNS mistakes, but either I'm blind/dumb (possible, DNS is not my strong suite), or it's something that is a bit more archaic. I've run out of ideas, and I can't seem to find anything that could explain why servers are unable to resolve the domains. There doesn't seem to be anything missing or incorrect.