You could just try to send an email via cli e.g.
# mail -s "Test" user@example.com
vzdump --mode=snapshot --quiet=1 --mailnotification=always --mailto=me@example.com --node=node1 --storage=backup --compress=zstd 101
Actually there is an option to test the notification system with Proxmox VE 8.1.3I also found myself in need of a way trigger a noteworthy event without actually bringing anything down. In my case, backups notifications are working, but not disk errors, they seem to be on separate channels.
PVE should include a way to test notifications. Nothing works until they are tested to be working.