What happens to mail when the server is down – is it returned to sender or lost?
Mail Transfer Agents (MTA) on the internet will continue to try deliver mail to your domain if your server is down, or if the domain is temporarily over quota or unreachable. Some MTAs will notify the sender if they are unable to deliver email within an hour but they will continue to retry to deliver the email. Most will continue to try and deliver for 3 to 7 days. If after that time, the email is still undeliverable, the MTA will give up and notify the sender of the failure.
