The hostname should be a domain name

Hi guys ... I have a problem already exposed but can't reply... so I open a new thread In mail-tester.com ... I have this errors... and I don't understand how to resolv. 1.999 FSL_HELO_BARE_IP_2 IP used in the HELO request The hostname should be a domain name, not an IP address If someone can tell how to fix please ??? When I connect with the terminal ... and command : "hostname" ... I don't have an IP but the domaine name. Can you please advice ? Thanks