I have a postfix email server up and running. My users have just been using webmail for the last 6 months. I have pop3 up and running now and I would like to get SMTP relaying for a few trusted I.P. address. Basically I am acting as an ISP for 3 other companies that sit behind static I.P. addresses. I do not see a need to configure a complete Cyrus-SASL authintication setup because these users will only be using the SMTP server from within these 3 companies. Would someone mind telling me the safest way to do this with postfix? Can I just safely add the 3 I.P. address of the remote sites to mynetworks?