Pure Message VS ES4000?? Are they the same?

Hi,

Basically we have the ES4000 appliance but we also have Sophos Enterprise console.

Pure Message is also available on our subscription which we have never installed but we are seeing a hug increase in spam emails getting through the ES4000.

I was wondering if Pure message is the same software running on the ES4000 Appliance?

Or does it work in a different way?

What I am trying to establish is.. Is it worth also installing Pure Message as well as having the ES4000 at the Gateway?

If they both work in the exact same way in theory any spam will be caught at the gateway and pure message will be doing nothing which makes it pointless installing it creating another process when email enters the building.

If it works different and there is a chance it will catch additional spam that the ES4000 unit has not caught then it would be worthwhile installing.

Then my final question is the ES4000 hardly ever picks up false positives hence why it is letting through spam how accurate is pure message? Does it pick up many false positives?

Thanks.