Local domains, listed in mydestination, are expected to have equivalent namespaces. That is every user valid in one, is valid in all.
If you want email addresses to not be tied to the shared namespace of
system accounts, configure the domains in question as
either virtual_mailbox_domains or virtual_alias_domains, and map
to system users indirectly.
http://www.postfix.org/ADDRESS_CLASS_README.html
http://www.postfix.org/ADDRESS_REWRITING_README.html
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Viktor.
