Linux Mail Server Setup and Howto Guide

Dear Admin,

Firstly, I want to say thank you for this forum because I found the solution that I want since last month. Actually I received thousands+ of spam email from only one domain/email address with different IP address and location eveyday. Until today I cannot solve this problem. But now I found in this forum "restrict sender access", because I also using CentOS 5 with webmin. I hope this method can really solve our spam email problem. But I don't understand using "postmap /etc/postfix/sender_access" command, when I type in remote mail server window the error message show "command not found". The forum say " Type in the command below in a terminal window to create a hash file." I don't what is it mean because I am very newest to this OS and mail server. Actually now I am monitor from remote computer (Indirectly) to manage the mail server CentOS 5 using webmin. During I tested all of the stages, the spam emails and all incoming email stop for awhile so I certainly rollback all of my action, I don't mean the steps not correct actually because of I am not really familiar with this one. Can you help me please?

Best Regards,

Horace