i got this postfix error when watchin maillog file (tail /var/log/maillog)

sendmail[21621]: My unqualified host name (*) unknown; sleeping for retry
My server hostname was like "vps" (command: hostname)
i was able to fix above error hopefully by doing command: hostname vps.mydomainhere.com
and then also adding that new hostname (vps.mydomainhere.com) into /etc/hosts:

myserverIPhere vps.mydomainhere.com vps

it was:

myserverIPhere vps

so it dont want one word hostname, but so called FQDN (fully qualiffied domain name)