Zentyal uses Postfix as it's Message Transport Agent (MTA). Normally legitimate e-mails from the Internet will always have a FQDN format. Local PC's or compromised PC's generally will just send the PC name. The restriction is contained in the configuration file /etc/postfix/main.cf and the restriction is reject_non_fqdn_helo_hostname. This file is created from a master config file main.cf.mas in the folder /usr/share/zentyal/stubs/mail. Always make a copy of any file you modify before modifying. I tend to create a copy with the date. e.g. cp main.cf.mas main.cf-29052018. In my version of Zentyal 5.1.1 the line to be removed is line 48 "$smtpRecipientRestrictions .= 'reject_non_fqdn_helo_hostname, ';"
After you have changed the line, restart the mail server and check that the reject_non_fqdn_helo_hostname is missing from the new file /etc/postfix/main.cf.
Hope this helps you.