These error messages has been reported by one of our users.
2018-09-29 08:42:18 H=(xxxx.xxxxxx.xxxxx) [yyy.yyy.yyy.yyy]:30000 X=TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=no F=<[email protected]> temporarily rejected RCPT <[email protected]>: Could not complete sender verify
It seems that some server is refusing the verification method that has been used by cPanel. To remedy this problem, log into WHM, go to Exim Configuration Manager. Search for Sender Verification Callouts and Sender Verification. Make sure to disable (set to off) those options.
With this kind of protection has been disabled, expect to got more SPAM in your Inbox. Since the actual remedy is to allow verification on sender server side (i.e: Sender SMTP server), you could consult to their SysAdmin for a fix if possible.
Note: There is an option to bypass sender using its IP Address. But it will be a lot of pain to manage this list.