Can anyone suggest me a proper tutorial to configure postfix SASL and tls in my centOs 5 server. I need this to authenticate clients on a different network to send mail from my mail server and prevent problem of open relay.
The official documentation for Postfix is fine for this:
You need to make a choice about whether to use Cyrus SASL or Dovecot SASL. If you are using Dovecot as an IMAP server on that machine, then use it (this allows you to use your Dovecot login details to authenticate with Postfix as well. Very convenient):
Otherwise, use the Cyrus SASL.
When using Cyrus SASL, you will probably want to use either the shadow or PAM plugin. Skip the others unless you know you need to use them instead (i.e. LDAP or SQL). You will need to install the SASL libraries. For plain login (i.e. PLAIN or LOGIN methods) you need:
Once you have configured SASL, follow the instructions for telling Postfix to enable SASL authentication.