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...should_send_now() function is called from from the "send_email_alerts" command. Returns false if specific subscription (called internally "email feed") should not be reported at the moment.
If you are not receiving instant alerts, it must be due to something else. A way to start troubleshooting is to create an admin account and set all subscriptions to "instant" in the user profile -> subscriptions. Then log in as other user and make a post.
If you are not getting a notification, the first place look into would the mail log file.