Postfix as relayhost with SASL auth

Client configuration (notebook, workstation what ever)

1. Edit your /etc/postfix/main.cf
2. Create your passwordmap - /etc/postfix/sasl_passwords
3. Run postmap for /etc/postfix/sasl_passwords

Server configuration

1. Nothing if you have already a working mail server. That the user (/etc/postfix/sasl_passwords) must exists on the server is plausible ? Check your smtpd_*_restrictions on the mail server if the client won't use sasl :)