establish policy for email services

we've had a few examples recently where I questioned support requests about email aliases, for example #40395 (closed), #40391 (closed), and especially #40378 (closed) vs #40348 (closed).

in general, the broad question here is, for email services when should we use one of those services:

In other words, there's a lot of stuff that can receive and forward email. When should we use which?