Absolutely this. At a previous company we got into a bad place with deliverability and ended up doing a load of work to repair it, several months of engineering

Absolutely this. At a previous company we got into a bad place with deliverability and ended up doing a load of work to repair it, several months of engineering time, moving domains, warming IPs, splitting types of email, building out better tracking, and ultimately, just paying the "deliver my email" racket fee to Microsoft.