[Postfix] Recipient address rejected: User unknown in virtual mailbox table

Hello,
I'm trying to setup my server (mx2.domain.tld) as secondary mail server. It should relay all emails from known domains to proper servers - in most cases mx1.domain.tld - server in LAN, but without ISPConfig installation).

On mx2.domain.tld I listed all known domains in postfix main.cf file:

Code:

relay_domains = domain1, domain2, domain3, ... , domain20
relay_recipient_maps =
mydestination = server2.domain.tld, localhost.domain.tld, localhost
relayhost =
smtpd_delay_reject = no
smtpd_helo_required = yes
smtpd_helo_restrictions = reject_invalid_hostname, reject_unknown_hostname, reject_non_fqdn_hostname
smtpd_client_restrictions = reject_unknown_client
smtpd_sender_restrictions = reject_unknown_sender_domain, reject_unverified_sender

Unfortunately I'm still getting such error message:

I also tried to use ISPConfig feature: Emails -> Global filters -> Relay recipients but it didn't work.