What happens when an email sent via SMTP cannot be delivered?

When an email sent via SMTP cannot be delivered, it is typically returned to the sender with a bounce-back message. This phenomenon occurs for a variety of reasons, such as an invalid recipient address, a full mailbox, or any kind of server error that prevents the successful delivery of the email. The bounce-back message is an automated response generated by the mail server, informing the sender that the email could not be delivered and usually includes the reason for the failure.

This process ensures that the sender is made aware of the issue so they can take appropriate action, such as verifying the recipient's email address or addressing other factors that may have contributed to the delivery failure. By contrast, storing emails in a queue for later delivery might happen in cases of temporary issues, but that typically refers to emails that are encountered with delay issues rather than outright failure. Deleting emails from the server or sending them to a spam folder are not standard practices for handling undeliverable emails.