There are many reasons why an email may bounce. One reason is if the recipient address is misspelled, or simply does not exist on the receiving system. This is a "user unknown" condition. Other reasons include resource exhaustion — such as a full disk — or the rejection of the message due to spam filters. In addition, there are MUAs that allow users to "bounce" a message on demand. These user-initiated bounces are bogus bounces; by definition, a real bounce is automated, and is emitted by a MTA or MDA.