Email not working after a few months

I have changed my Cpanel email dns to Cloudflare. It worked without any problems for a few months until today. Now I can access my email directly from a web browser. But email clients like Outlook or eM Client don’t work. It gives “Server not responding” error. What am I missing here?

Assuming they’re connecting to the mail record then Cloudflare shouldn’t be doing anything to the traffic - the record being DNS only means that all clients connect directly to the IP address in question as opposed to being sent to Cloudflare like a Proxied record would.

Have you checked on your email server itself?

Well it turned out that the firewall new policy caused this problem. So I changed the rule and it is working again.