If so, how do we diagnosis and solve the problem?
Cloudflare is not proxying that hostname (the imap subdomain), so CF isn’t the source of this. You need to make sure that port is forwarded/open on your server.
CF is used for the proxy SSL certificates. The hostname for the server is imap. (domain name) but this subdomain is a special subdomain for the admin6 which is a cPanel account. Do you recommend a different hostname for the server to make sure the server’s service SSL certificates are not confused with the account service SSL certificates?
$ dig imap.cfored.com +short 220.127.116.11
That hostname doesn’t have CF IPs, meaning it is not proxied. You can change from “DNS only” to “proxied” by changing that subdomain in the DNS tab.
A smart host server funnels SMTP mail traffic from CF to the mail client imap.( subdomain name). If the subdomain is proxied, then smtp traffic is funneled to the wrong IP address.
Issue resolved by deletion and creation of new cPanel firewall port rules. Thanks