"When sending mail, the Postfix SMTP client can look up
the remote SMTP server hostname or destination domain (the address
right-hand part) in a SASL password table, and if a
username/password is found, it will use that username and password
to authenticate to the remote SMTP server. And as of version 2.3,
Postfix can be configured to search its SASL password table by the
sender email address."