@lightweight The big question is deliverability. When I've tried this, I've ended up spending lots of time trying to convince -- sometimes successfully, sometimes not -- large providers that the emails coming from our service were not spam (which they 100% were not, by the way). The burden of those interactions was very high, but the price of not engaging in them was that our users could not send email to anyone with, e.g., a Hotmail address.

How did you solve this? Or do you not have this problem? Or do you have it but tolerate it?

We knew about SPF/DKIM/DMARC, and AFAIK implemented everything correctly. I'm sure that helped -- our delivery rates would have been much worse without that. But still, some providers are just eager to reject, and yet sometimes people you want to reach are using those providers.