Add the following to your main.cf file (/etc/postfix/main.cf)
smtp_bind_address = 192.0.2.3
to set tell smtp to bind to this IP (smtp_bind_address6 if using IPv6).
note that with this smtp will always use this IP (contrast with the
default when smtp binds to ‚Äúany address‚Äù and it is the routing code that
selects the source IP based on the route (outgoing interface).