By default smtpd_reject_unlisted_recipient is set to NO. If I send an email to an account that does not exist such as test@example.com then the message bounces.

I found this,

smtpd_recipient_restrictions = reject_non_fqdn_recipient, permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination, reject_unlisted_recipient, reject_invalid_hostname, reject_non_fqdn_sender, permit

which must be the reason why the email bounced, so why are there posts on this forum where users set smtpd_reject_unlisted_recipient to YES? Isn't this redundant?