From Postfix documentation:
With this setting, the Postfix SMTP server will not reject mail with "User
unknown in local recipient table". Don't do this on systems that receive mail
directly from the Internet. With today's worms and viruses, Postfix will become
a backscatter source: it accepts mail for non-existent recipients and then
tries to return that mail as "undeliverable" to the often forged sender
address.
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