If a message cannot be delivered, you will receive an email back from the last system who could handle the message, called a bounce message. A bounce message has a few standard parts:
- It's (usually) sent from Mail Delivery System, Mailer-Daemon, Postmaster or root@ the system who last handled it.
- It shows the email address it was trying to deliver to, and the error it received.
- It shows the message as it looked when it attempted delivery.
- A well-formed bounce message will also show you the name and IP address of the system who gave the error.
With this information, a bounce message can actually tell you quite a lot! For starters, it tells you why a message couldn't be delivered. Nowadays, the most common ones you see are either "User Unknown" (which means the mailbox has been shut down), or an error relating to spam. Less frequently, you may also see messages about "User over quota" or "Connection timed out" or "message too big". All of these can tell you why your message wasn't delivered to your correspondent.
Other elements can provide information, too. For instance, if the email address in the bounce doesn't match the email address you sent your message to, they could be forwarding their mail (and there could be a problem with that.) The message you sent could also include headers that indicate the path a message took from you to your correspondent. The name and IP address of the system that gave the error sometimes tells us that the system that's handling the mail isn't the one you'd expect. All these details are why, if you indicate that a message bounced, that we ask you for the full bounce message (not just the error.)