if your ISP:
1. disallows other SMTP values, and
2. requires authentication
... then it means their servers cannot be used to send anonymous spam. of course, you could send spam, but they have your credentials.
i don't know why it's a big deal to not be able to do it. in Mail.app, for each of your email accounts, whatever they are, just set the SMTP value to what your ISP says you should use. you can still brand your emails by selecting the from address.