We have a mail gateway running postfix and MailScanner for our inbound mail, it forwards on to exchange.
All SMTP traffic comes in here.
It allows relaying for any domains that it is supposed to, and only relays those to our exchange sever.
I have roadwarriors that are using imap/pop for inbound, and that's pointed directly to exchange. But I want their outbound email to point to the postfix box, and authenticate them with a password.
I've created a user account for them (actually, a common account, it's only a handfull of people) but I need to know how to tell postfix to allow valid users to send.