How sending email works / SMTP

How the email process works:
Suppose the mail server is unreachable at some point, for whatever reason. What happens to email that is sent to your domain?
Whoever sends the email always does so via the SMTP protocol. This protocol follows certain standards.

If the email cannot be delivered, delivery will always be retried in stages. Usually there is a new attempt after 15 to 30 minutes, another after 60 minutes and then often every 2 hours for a period of at least 2 to 3 days. These settings can vary slightly per mail server. If the email still has not been delivered after those 2 to 3 days, it is bounced (returned) with the reason included. The sender may also be informed in the meantime (every 24 hours) that the message has not yet been delivered (this differs per mail server).

So if the mail server has been down for a while, you can rest assured that the email will still arrive (later) once it is back up, and that the sender will in any case eventually be notified if it cannot be delivered. In principle, emails cannot simply disappear.

The same applies when you send email via our servers yourself. If it cannot be delivered, you will be notified.

So if email is delivered later because a mail server was down, it may not be immediately obvious to you that this is an email sent earlier, because most email programs show the date received and not the date/time the email was sent. In some email programs you can also display the date sent; otherwise you can always view the headers of a message (message source) to see the original date/time it was sent.