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:
email@example.com (Which is set as the catch-all)
Now in this example, any email sent to firstname.lastname@example.org would be delivered to that corresponding mailbox.
The same would hold true for any email sent to email@example.com.
However, what would happen if an email was sent to “firstname.lastname@example.org”?
Because the mailbox email@example.com 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 ‘firstname.lastname@example.org’
This would hold true for email sent to any other addresses which have not been setup as mailboxes on the system (ie email@example.com)
We sometimes also refer to this email address as the Default Email.