I had the same issue.
After some digging, here is what I found:
CWP Dashboard -> Email -> MailServer Manager
Fix your Current Settings in Postfix:
mydomain = mydomain.com
myhostname = websrv.mydomain.com
(change the names to match your information)
Don't forget to check:
✔AntiSpam/AntiVirus (recommended)
✔rDNS Check (recommended)
✔Install DKIM & SPF (recommended)
✔Install Policyd (recommended)
Then, "REBUILD MAIL SERVER" .
Depending on the speed of your machine, I had to wait a good 20 min, maybe more.
After that, I checked:
CWP Settings -> Edit settings
make sure you have set an email in the = CSF/LFD Alerts
Then I rebooted the server, check email settings,s and the CWP dashboard (CWP info) box for "Hostname".
Fixed
Hope this helps.