Third Party RBL when SMG is deployed after an MTA

Howdy folks,

Symantec recommends that the SMG be installed on the gateway before all other MTAs and people at my work place decided to go aganist that for some reason.

As a result we are always under spam fire.

I am considering using third party RBLs now for further blocking SPAM right at the gateway, and needed an explanation as to whether the SMG deployed after the MTA would still be able to detect the incoming blacklisted IP address.

As of now the message audit logs shows that all our email comes from just 2-3 IPs which are the IPs belonging to our SMTP providers IP.

Thanks for reading.

Appreciate any input.