A catchall address is a designated mailbox where any email sent to a non-existent email address will be delivered. To better help explain catchalls, review the example below:
You have the following mailboxes setup on your account:
firstname.lastname@example.org (Which is set as the catch-all)
Now in this example, any email sent to email@example.com would be delivered to that corresponding mailbox.
The same would hold true for any email sent to firstname.lastname@example.org.
However, what would happen if an email was sent to “email@example.com”?
Because the mailbox firstname.lastname@example.org has not been setup in the system, normally the email would be rejected with a “user unknown” message and would be returned to the sender.
However, if you have a catchall setup, like in the example above, the email would be delivered to the catch-all mailbox which in this case is ‘email@example.com’
This would hold true for email sent to any other addresses which have not been setup as mailboxes on the system (ie firstname.lastname@example.org)
We sometimes also refer to this email address as the Default Email.