Unable to send or receive e-mails from remote mail addresses.
Customer in Plesk is unable to access Plesk Email Security extension, the following error message is displayed:
Hostname is not valid. Please set a hierarchical, dot-separated domain as the hostname!
The server hostname must be a fully qualified domain name (FQDN). The hostname may either be incorrectly configured in Tools & Settings > Server Settings > Hostname or via hostname command.
Go to Tools & Settings > Server Settings and set a Full hostname.
If hostname already set, change it to some random one (ex. test.example.com) and then change back to correct hostname. This will trigger the change to
Open Plesk Email Security in Extensions > My extensions > Plesk Email Security > Open
Click the button Install Now!