Here a quick run down on how to block specific domains in postfix.
First edit the /etc/postfix/main.cf file
Add the following line:
smtpd_sender_restrictions = hash:/etc/postfix/deny
next create the deny file
/etc/postfix/deny (this is where you will put your list of domains)
Use this format:
Now use port map to that Postfix can read the file at startup.
Now restart postfix
service postfix restart
Send a email from the domain… Yahoo for example to test, look at the log file.
Note that after you add to the deny file you will need to use the portmap command again and restart postfix.