When a message is sent to a mailing list, the mailing list software re-sends it to all of the addresses on the list. The presence of invalid addresses in the list results in bounce messages being sent to the owner of the list. If the mailing list is small, the owner can read the bounce messages and manually remove the invalid addresses from the list. With a larger mailing list, this is a tedious, unpleasant job, so it is desirable to automate the process.