Unable to send or receive e-mail messages: Client host rejected: cannot find your hostname

Symptoms

• 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:

  PLESK_ERROR: Hostname is not valid. Please set a hierarchical, dot-separated domain as the hostname!

•  

Cause

The server hostname must be a fully qualified domain name (FQDN).

Resolution

1. Log into Plesk.

2. 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.

3. Open Plesk Email Security in Extensions > My extensions > Plesk Email Security > Open

4. Click the button Install Now!

Additional information

Server with Plesk Email Security no longer sends/receives emails: [127.0.0.1]:10024: Connection refused