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If mails get transparently forwarded to another mailserver, a mechanism to block mail for invalid recipients makes sense, and drastically increaes the well-known backscatter problem.
LDAP queries are used to check for valid recipients, and forwards the mail, if an entry for the user is found.
For this to work, on Debian/GNU Linux, you also have to install postfix-ldap by
apt install postfix-ldap
Further information can be found @ https://blog.nwsec.de/wordpress/?p=1031
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