I recently started using CloudFlare and am having a few teething problems.
Our domain is netlanguages.com and while we have a lot of sub-domains listen, we are currently only trialling a few of the servers through the CloudFlare CDN (for example, www.netlanguages.com is enabled for CDN, netlanguages.com is not). The actual CDN service seems to be reliable, but the problem that we are having is with DNS, and specifically with mail delivery.
The background is that we have contact forms on our web site which use PHP mail() to send the details to end-users' email addresses, with the "from" address of the messages being firstname.lastname@example.org which is a valid address on our mail server. Most of the mails are arriving correctly, but a few specific people are not receiving them. The webserver uses qmail to deliver the messages, and the qmail log files show us some of the errors that the receiving mail servers return when they reject the mail delivery attempt. Two examples:
Connected to 220.127.116.11 but sender was rejected./Remote host said: 421 DNS problem (interdominios.netlanguages.com). Try again later Connected to 18.104.22.168 but sender was rejected./Remote host said: 451 DNS temporary failure (#4.3.0)
From what I can tell, the receiving SMTP server is doing a DNS lookup of some description on either the host of the "from" email address (netlanguages.com) or the server name given in the EHLO command of the SMTP conversation (in the first example above, interdominios.netlanguages.com), both of which should resolve to non-CloudFlare IP addresses.
I've read that the CloudFlare DNS service is very reliable and fast but both of the problems above seem to point to a problem with remote servers unable to do DNS lookups.
I should also point out that we changed our DNS to CloudFlare on 6th Feb, and since then started experiencing these mail delivery problems. On 22nd Feb we moved our DNS away from CloudFlare to see if the issues were related to CloudFlare and after a few hours delivery began to work. Then on 26th Feb I moved the DNS back to CloudFlare again and delivery problems started again. The issues definitely seems to be related to DNS, but I don't know if it's a configuration issue, or something else.
Finally, I should say that our two DNS MX records point to non-CDN A record IP addresses, interdominios.netlanguages.com (the web and qmail server) also points to a non-CDN A record IP address.
Does anyone know what the problem could be here? Any light you can shed on this will be most appreciated.