If the receiving server, domain, or mailbox name does not exist, then you will receive a bounce-back message (or possibly no message at all). If the receiving server is deferring or queuing emails (because it is too busy or broken), then it just depends on the mail system as to how long it will keep waiting. If you are the receiver, our Exim mail system will hold emails up to 7 days. If you are the sender, our gateways will hold emails for up to 2 days.
Email server unavailable for delivery.
