To remove postfix from sending an email to a user with an unknown email address, the header_check line has to be uncommented in the postfix main.cf configuration file.

In the main.cf file, the header_check line should look like this:

header_checks = regexp:/etc/postfix/header_checks

The /etc/postfix/header_checks file will have the following as its content:

/someunreachableemailaddress@yahoo\.com/    DISCARD