How are you sending behavioural emails in your Saas?

So far me and my co-founder have explored building a system internally to trigger and send emails based on user behaviour, as well as use a newsletter platform like Beehiiv.

I wondered what are the ways IndieHackers are sending emails based on user behaviour in-app. For example: credits running low, prompting an upgrade after high usage, getting towards the end of a trial plan, following up after subscription cancellation, and so on.

There must be a solution for this?

Cheers!