I never said anything about monitoring outbound SMTP traffic.

The more realistic mitigations are e.g. periodic scanning for open relays, actually handling abuse email reports, RBL checking

A provider that isn’t on the ball about managing outbound spam will quickly find their IPs (if not the whole prefix) blocked. If someone runs a spambot from a VPS, and then you get the recycled IPv4 address when the instance is removed, what’s to tell Microsoft you’re not also a spammer?

Assuming you’re using postfix as an MTA, it’s not too hard to get OpenDKIM up and running.

My mail admin experience is pretty dated these days, but I followed this guide back in the day and had it working.

“This tree seems really sensitive, I ran a chainsaw through it and it just fell over???”