If you have access to the root server with ssh then do this:
Disclaimer: This will have result on the entire server, not just for one domain like you asked for.Edit postfix main.cf and add this under smtpd_sender_restrictions
check_sender_access hash:/etc/postfix/sender_access
Create file '/etc/postfix/sender_access'
Add this to the file:
/\.TLD$/ REJECT
or
/\.TLD$/ 550 Blacklisted
The second one will send a "550 Blacklisted" error message to the sender
Then run command:
postmap /etc/postfix/sender_access
then restart postfix
This will
block the TLD from ever getting the message sent to your server.
ORif you want for one domain/email account, I advice opening up the mail account with roundcube and edit the filter from there. The filter manager on CWP has been buggy in the past (user/file permissions issues), though I'm not sure if it's been fixed.
Here's an example:
and change actions to "delete message".
This will not block it, but it will delete it. Filters don't block. Filters works because it first has to receive the message to know what to do with it.