Q: How do I opt for email notice when someone replies

You'll need to change your spam settings to change that. Whitelist the address used to send the message, which I think is: discussions-updates@apple.com

tt2

If you use any robust email client like Apple Mail, this general article FOR Apple Mail may help

Mail (Yosemite): If junk mail filters aren’t working correctly

Your binternet WebPageMailClient may also have a simple Button labeled [Not SPAM/Junk] that would let your email provider "learn" about mail from ASC notices

As CCC alluded to, this is being done by your ISP, not on your device or computer. You need to login to your mail in a browser, navigate to the spam folder, and there should be a button to mark one of the emails "not junk". That should be enough to whitelist the sender.