This is because the server has no FQDN set, and only has shortname. SMTP EHLO will fail if FQDN is not available.
Following is an example of host without FQDN
Please set a FQDN for your server using hostnamectl. If you don't have domain for your environment, just use example.com or localdomain as your FQDN.
hostnamectl set-hostname qorestor.example.com
Rerunning test email alerts will be successful after that.