CIDR format table in Postfix doesn't work.

Issue

I am currently building a relay-configuration which will need /etc/postfix/network_table file, this file is expected to have up to 500 to 1000 entrys of subnets and ip's. The table is needed to restrict access to the relay-server, the system will be the relay server for several large sites (internal sites only).
We need to restrict which ip's/subnets are allowed to sent mail. This is to prevent random IP's and/or end users from sending mail via the relay-server.
Getting following error while trying to run postmap on CIDR table

# postmap cidr:/etc/postfix/network_table
postmap: fatal: unsupported map type: cidr

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