Using namecheap dns with cloudflare

It's not really a CNAME, though, is it? It's a configuration of where they should pull your site from when they get requests for it.

CloudFlare's service directs traffic to their own servers as a caching proxy. Their example has a CNAME going to an Amazon EC2 address, but that's not the server they're going to point you to when your client queries for it; instead, they'll return the address of their own proxy host in response to your query.

If they did indeed configure a CNAME record on your root domain, it would indeed break your MX and SOA records - but I suspect that's not how they've implemented it.

First, thanks a lot for using your actual hostname and information in the question, it helps a lot!

The 451 DNS temporary failure means that the recipient mail server is unable to resolve the hostname at that point in time. I've looked at your DNS records and I can't see anything strange about them that would cause this, so I'd say that the problem is with the CloudFlare DNS servers. It may be that there is some firewall or rate limiting or some other issue that means they either don't respond or don't respond quickly enough for the mail servers to get the information they want in order to accept the emails.

You can contact the postmaster at the recipient site (in theory, the address "postmaster@theirdomain.com" should both work and be read by a human being, but we all know the difference between theory and practice!) and ask them if they can see why the checks failed. I'd also check with the CloudFlare support staff if they've seen the same problems for other customers.