Postfix - specify interface to deliver outbound mail on
Solution 1:
You're looking for smtp_bind_address and smtp_bind_address6. Details are available here.
Solution 2:
Would inet_interfaces do what you want?
When inet_interfaces specifies just one IPv4 and/or IPv6 address that is not a loopback address, the Postfix SMTP client will use this address as the IP source address for outbound mail. Support for IPv6 is available in Postfix version 2.2 and later.