Proxmox Notification Email Error
Proxmox is using postfix as email software, and the configuration of sender email uses local hostname+domain name as below in
myhostname=full qualified hostname
The postfix tries to look for email receiver's email server, and send emails directly to the email server using
myhostname defined in
/etc/postfix/main.cf as senders DNS name. But the server hostname normally isn't using the public valid domain name, which causes email server rejects notification emails.
Change the default
/etc/postfix/main.cf to a valid public email domain name.