Best practice for out-of-app notifications for in-app messages?

Platforms like Task Rabbit and Airbnb send users an email and/or text message when an in-app message is received.

What is best practice in terms of UX for how and when these out-of-app notifications (e.g. email / text message) are sent?

One strategy could be to simply send notifications every time a message is received in-app. But I wonder if that could bombard users who receive a flurry of quick notifications, especially if those notifications are via text message or email. A counter argument to that could be that many message and email clients tend to group together emails from the same sender nowadays, so the recipient may not feel bombarded at all.

I thought about more nuanced approaches, where out-of-app notifications are only sent if certain conditions are met. For example max 1 notification every 30 seconds. But I'm not sure if that's a good idea.

So in terms of medium, frequency, and any other considerations, what are best practices for notifying users of an in-app message?