smtp_bind_address (default: empty) An optional numerical network address that the SMTP client should bind to when making a connection.
This can be specified in the main.cf file for all SMTP clients, or it can be specified in the master.cf file for a specific client, for example:
/etc/postfix/master.cf:
smtp ... smtp -o smtp_bind_address=11.22.33.44
Note: when inet_interfaces specifies exactly one address that is a non-loopback address, it is automatically used as the smtp_bind_address. This supports virtual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces documentation for more detail.