Some people play with VMs for fun, or as a leaning experience. I don’t think it’s very productive or useful to tell them they’re doing it wrong.

If you find yourself needing more CPU horsepower, try the 5. If you don’t, stick with the 4.

I self host my own mail, SMTP in and out. But I’ve also had this vps in the US from a small local hosting provider for a decade. Unfortunately vps providers can be a crap shoot if you’re looking for a new one. My back up plan is this one goes south is to just use a cloud provider’s SMTP relay service (eg aws) because their prices for that sort of thing are extremely reasonable for small volume.