Re: Smarthost likely required
The Postfix docs cover quite a bit about these issues. Setting up a host for outbound relaying of all mail is simple:
relayhost = [mail.isp.com]:587
would send via port 587 on your ISP, for example. It's also quite easy (described in full here) to use SASL on that connection.
You can, if you want to get really fancy, have different outbound hosts for different email, and differed SASL credentials for each person too.