When sending an email to a non-existent user (i.e. email@example.com), which is in
Subscription domain, the email is forwarded to an external address (i.e. firstname.lastname@example.org) as per the 'What to do with mail sent to nonexistent users' option under the 'Mail settings'.
Additionally, if the email is sent to a non-existent user (i.e. email@example.com), which is in additional domain, NOT in the Subscription domain (i.e. example.com), a bounce back with the following error is received:
Recipient address rejected: User unknown in virtual mailbox table
"catch-all" address is not set for the additional domains.
Update bounce settings for the affected domains by executing the following command:
# /usr/local/psa/bin/subscription_settings -u <affecteddomain.tld> -no_usr <firstname.lastname@example.org>