smtp_bind_address (default: empty)
An optional numerical network address that the SMTP client should bind to when making an IPv4 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 1: when inet_interfaces specifies no more than one IPv4 address, and that address 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.
Note 2: address information may be enclosed inside [], but this form is not recommended here.