These are email addresses with mailboxes on a domain capable of “catching” all the emails sent to the same domain. It doesn’t matter if the address exists or not. Catch-all emails are commonly used by government and business organizations to ensure they catch all the emails headed to their domain.
If you send an email to misspelled recipients at your domain, such emails are “caught” by the catchall account. Here is an instance – if the catchall is defined as [email protected], and an email intended for [email protected] is sent, misaddressed, to [email protected], such an email will land in the catchall mailbox of [email protected]. If the catchall wasn’t defined, the email would have ended up bounced back to the sender.
So, catchall works as if it gives you unlimited aliases. However, the downside here is that you may receive more Spam to your domain, after all, messages are accepted automatically to non-defined email recipients rather than getting bounced.