Following various "how to" articles here and elsewhere, I've been building a mail server. The information about domains and users is held in MySQL. Courier uses the user and password information to handle logins. However, I cannot figure out how to deal with Postfix user/password validation. When a mail client is configured to use an SMTP server to relay, it is nowadays normal to enter a username and password. But my server is rejecting passwords. This isn't altogether surprising since none of the Postfix related .cf files I've seen has any mention of passwords. Am I on the wrong track? What needs to be done to configure the server to accept user names and passwords and to permit relaying to users who provide valid credentials?