You sent an email, and it was bounced (returned) back to you.
If you see a black screen initially, just click into the VNC window and press any key (e.g. Space, CTRL or Enter). The operating system should show something now.
Usually, you receive an email with the subject of this article from your cPanel system because you have installed a package using a third-party repository.
During the boot of Ubuntu, you read the message "No symbol table found."
Whenever you reach the limit of any particular resource, either it is RAM, CPU Usage, or a high I/O Usage, the load average of your server will get higher, and processes will start to wait.
If you can use the control panel to manage your service, connect via SSH or VNC, most likely, it is not a DDoS attack.
Check the error log file for errors. Coding errors on your website are most likely causing the blank pages.
Resource Limit Reached is a message that may appear when you have Cloudlinux installed on your server.
Confirm if you are using the correct parameters to connect your web application with the email account.