3.7 How can I secure Postfix so it won't answer verify requests?

You will really like the fact that Postfix, by default, disables verify. A prospective spammer is forced to send mail to ascertain whether a user exists on your system or not. Here's an example run against a default install of postfix-19990906_pl07-1:

   [root@boss SPECS]# telnet localhost 25 
   Trying 127.0.0.1... 
   Connected to localhost. 
   Escape character is '^]'. 
   220 boss.somewhere.org ESMTP Postfix PostFix-v.2000 
   helo localhost 
   250 boss.somewhere.org 
   vrfy tom 
   252 Send mail to find out 
   quit 
   221 Bye 
   Connection closed by foreign host.