From Seo Wiki - Search Engine Optimization and Programming Languages
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In common use, a catchall or catch-all is a general term, or physical dumping group.
In the context of e-mail, a Catch-all usually refers to a mailbox on a domain that will "catch all" of the emails addressed to the domain that do not exist in the mail server. Configuring a catch-all address can help avoid losing emails due to misspelling. However many email hosts no longer permit catch-alls since many spammers abuse the feature, spamming random email addresses.
For example: if firstname.lastname@example.org is configured to be the catch-all email address on example.com, then any email sent to an undefined @example.com email address will be forwarded to email@example.com instead of being rejected as an Undeliverable message or Unknown User error.