Postfix: Recipient address rejected: User unknown in local recipient table, even though recipient domain is an external domain

The parameter that's you should aware of is parent_domain_matches_subdomains.

A list of Postfix features where the pattern "example.com" also matches subdomains of example.com, instead of requiring an explicit ".example.com" pattern. This is planned backwards compatibility: eventually, all Postfix features are expected to require explicit ".example.com" style patterns when you really want to match subdomains.

So, parent_domain_matches_subdomains holds the list of domain that postfix should match its subdomain too even without (dot) in front of it.

Unfortunately, the relay_domains parameter fall to that list.

# postconf parent_domain_matches_subdomains
parent_domain_matches_subdomains = debug_peer_list,fast_flush_domains,mynetworks,permit_mx_backup_networks,qmqpd_authorized_clients,relay_domains,smtpd_access_maps

And because your relay_domainscontains $mydomain or central.mydomain.tld, postfix matches all subdomain of central.mydomain.tld as relay_domains not external ones.

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The solution is set parent_domain_matches_subdomains and don't include relay_domains on those parameter.