I noticed that if a user had a email-address created with the name "shadow" - like "[email protected]" the email-address will not be listed in the users cpanel after an upgrade to Cpanel11.
Further more no emails can be delivered to a email-address with the name "shadow" in it.
No forwarders works either - like example "[email protected] --> [email protected]"
And if You have "catch-all" set to forward emails to for an example "[email protected]" all emails sent to for an example "[email protected]" will not be forwarded.
What happens with the mails then? Will they bounce back to the sender? Well NO - they will be stranded in the mailqueue first - often for several days. If You try manually to deliver them an error-message like this will occur:
A perfect opurtunity for spambots or "mean" people to fill up Cpanel servers with thousands of emails addressed to "[email protected]". 
Further more no emails can be delivered to a email-address with the name "shadow" in it.
No forwarders works either - like example "[email protected] --> [email protected]"
And if You have "catch-all" set to forward emails to for an example "[email protected]" all emails sent to for an example "[email protected]" will not be forwarded.
What happens with the mails then? Will they bounce back to the sender? Well NO - they will be stranded in the mailqueue first - often for several days. If You try manually to deliver them an error-message like this will occur:
Code:
== [email protected] R=virtual_user_filter defer (-1): require_files:
> error for /home/mydomain/etc/mydomain.com/shadow/filter: Not a directory