Hello, I see that the configserver security is creating the problem, I disabled it and now the emails are being delivered to the customers properly, how to enable default php mail on CSF to avoid the issue on SMTP plugin, please help me here brother. everything else works fine. actually you are my guru you taught me everything in terms of setting up the server and fixing the email issues in the past, thanks and I appreciate your time and patience 
Any resolution here brother?
how to exclude PHP mail from cyberpanel?
Figured it this out, thanks for your help.
Would you mind sharing how you resolved this so that it can serve as a guide for someone that might have similar issue in the future.
I wasnāt using the service that time so I didnāt realize that there was an issue. Iām wondering, is the issue already resolved? I really hope it was since itās such a hassle not being able to use it properly!
Got to CSF page then click on option āFix Common Problemsā - Then click on āDisable SMTP_Blockā option. That should fix your issue. that will do the trick