On a newly installed control panel you might have this situation:
Hostname: example.ispdomain.com
Client domain: example.com
This might cause problems with MX Toolbox’s health check here:
https://mxtoolbox.com/emailhealth
The lines in main.cf to look at are:
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
myhostname = example.com
mydestination = $myhostname, example.com, localhost.co.za, , localhost, example.ispdomain.com
Change myhostname to the correct banner to avoid this issue.
Also make sure the reverse DNS is set up properly.