Two suggestions for a starting guide. Dated, but still helpful, Dan Langielle's FreeBSDdiary page
http://www.freebsddiary.org/postfix.php
Other good one, Ned Slider's article on the CentOS wiki
http://wiki.centos.org/HowTos/postfix
Very brief, but to the point explanation of typical main.cf changes. Those won't be Linux/BSD specific, they're just postfix specific. (Much of the rest of it is about installation on CentOS, and configuring Dovecot.)