This isn't CCed but you get a mail. If you need to changes this often and have a lot of cluster hosts you could also place a file with the list of address into /etc/pve (synced over cluster-nodes) and use the ":include:/file/name" directive, see
man 5 aliases
for some more info.
But IMO, it could be a valid feature to allow marking some users (admins) as email recipients for all mails send to the nodes root@localhost adress...
And yes - agreed, it might be nice to make this a feature request for the Web UI somewhere - just a text field that lets you put in CC addresses. How do we file those?
I see that Proxmox mail works out of the box - I assume just using SMTP on the localhost. However, it might be nice to use something like Sendgrid or Mailgun for more guranteed delivery. (Some hosting providers block outgoing mail, or your mail may get flagged as spam).
Could a second feature request be for an easy way to configure our own SMTP server, with username/password, and ports etc?
echo -e "Subject: This is a test\r\nA test message to see if forwarding works to all specified addresses.." | sendmail root@localhost
This is, in principle, the same thing we do when we want to notice the root admin, we send a mail to root@localhost, which then gets handled by our mailforwarder (it pulls the current user-configured mail, adds a To: and From: part and sends it really out with sendmail)
But, this simple test will highly probably land in your spam queue, it misses a From: and To: header, and depending on your domain/postfix is send out in a bit of an "sketchy way".
So check those spam folders if you do not see it in your inbox..