Most (properly configured) MTAs will queue and retry, so any unsent mail shouldn't be blindly discarded. Postfix, for example, defaults to a queue lifetime of 5 days.

Potentially not, many MTA relays will retry a couple of times before completely giving up.

The sender might also get a delivery failure message from the relay.

I was just wondering about this. Does anyone know what happens to e-mails sent to domains that aren't resolving? Do they really just get lost?