If you ISP provides SMTP for their e-mail (almost all do), then barring outbound From address filters, you can set up your SMTP server to treat the customer facing mail server as a smarthost.
The ISP doesn't need to do anything specific to support it.
I was absolutely sure they did filter, but I've just tested and apparently they don't. Well, that's unexpected. I might switch to self-hosting again then.
The ISP doesn't need to do anything specific to support it.