I think you're overstating how much private mail servers are penalized. I've run several on colocated machines, and delivering emails to the major providers has

I think you're overstating how much private mail servers are penalized. I've run several on colocated machines, and delivering emails to the major providers has never been a problem. You have to make sure that your colo gives you IP addresses that aren't on any blacklists, but other than that it's fine.