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In common use, a catchall or catch-all is a general term, or metaphoric dumping group, for a variety of similar words or meanings. For instance, a catch-all taxon a.k.a. "wastebasket taxon" is a grouping of organisms not fitting anywhere else.
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.
For example: if email@example.com 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 firstname.lastname@example.org instead of being rejected as an Undeliverable message or Unknown User error. Therefore, if the sender misspelled it "conttact" it would be automatically redirected to the correct address.