Email delivery failures generate error messages from the mail server, known as bounces. These messages provide details about why the email couldn't be delivered.

There are two types of bounces: soft and hard bounces.

Soft bounces occur when emails can't be delivered at the time of sending, even though the address is valid. Reasons might include a full inbox or temporary unavailability of the recipient's server.

Hard bounces happen when an email address is invalid (logically incorrect) or doesn't exist. The recipient server automatically sends a message that the MAILINGWORK server reads and interprets. Hard bounces may also occur if the provider is temporarily unreachable. For this reason, you should only clean up bounces after two or three consecutive occurrences.

Learn why it's important to regularly remove hard bounces in this article: Why Should I Remove Hard Bounces?