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In common use, a catchall or catch-all is a general term, or physical dumping group for a variety of similar words or meanings. For instance a catch-all 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. However many email hosts no longer permit catch-alls since many spammers abuse the feature, spamming random email addresses.
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.
However, since the advent of mass spamming, it is considered better practice to refuse all emails with unknown addresses, to help combat spam and reduce network traffic. Email addresses that are often misspelled may be added as an alias, for example "stephen.bloggs" and "steven.bloggs".