i believe the setting is default email mailing list mode frequency and if you set it to “send email for every new post” they get notified of their own posts (the other option is “send an email for every new post except my own”).
users can also set this in their own preferences (with “send me an email for every new post” or “send an email for every new post except my own”) in their own mailing list mode, so i think it can be overridden there.