Step 1.Open /etc/postfix/main.cf & enter below line at end
transport_maps = hash:/etc/postfix/transport
Step 2:Open /etc/postfix/transport & need to insert line specifying which domain should be allowed as below:
test.com :
* discard:
Step 3:Now need to create a hash of the file
postmap /etc/postfix/transport
Step 4:Finally reload or restart postfix and you are done.