I wanted to do some testing with some forms on my web site, but I did not want to deal with the resultant output going to email.

I added the following line to the /etc/postfix/transport file

fubar.com   discard

Then I added the following line to the /etc/postfix/main.cf file

transport_maps = hash:/etc/postfix/transport

Then I executed

postmap /etc/postfix/transport
systemctl restart postfix

Any email destined to (anybody)@fubar.com gets sent straight to the bit bucket.

Tested on Centos 7