According to RFC 2821, but also to Internet best practices, a mail server must be FQDN. An FQDN server is a fully named server on the Internet: Fully Qualified Domain Name. This is a full host name, for example: “server.domain.tld”. The maximum length of an FQDN name is 255 characters. The FQDN name is used to uniquely identify a server on the Internet.
A mail server that isn’t FQDN is presented using a simple name (“server” in our example instead of “server.domain.tld”). This type of workstation generally corresponds to user workstations, but a user workstation that sends emails directly to the outside world is a zombie workstation. A user workstation always uses a mail server to send e-mail messages.
Like 80% of the world’s mail servers, ALTOSPAM systematically refuses emails from non-FQDN servers. In this case, it rejects for “HELO invalid”. A server that is mistakenly not configured is unable to send emails to most of its contacts. To solve this problem, all your sender has to do is define his server name in full by adding his domain name in /etc/hosts (or equivalent), and restart the mail service.
The following article takes a different approach to the problem raised in this article