Any recommendations for transactional user notifications? (i.e. email, text, push, in-app, etc.)

Hey IH! Does anyone have a recommendation for a transactional user notification service?

I need to send transactional notifications to users of a SaaS product, and I'd like to offer multiple channels – email, text, push, in-app, etc. I'd like to send through just one service instead of having to send through a bunch of different services for each channel.

Any recommendations or thoughts are much appreciated!