I cannot block an IP address

I'm baffled. I have a rogue process on a web server that I have no control over sending tens of thousands of email messages to my server. I created what I thought was a very simple packet filter rule that says the source is this IP host, destination smtp (I tried with any), destination is any, and I've tried drop and block. The connections are still allowed. The rule is at the top and is turned on.

It would seem to be a simple thing to do, but I'm not having any joy. What am I missing here?

Thanks.

Danita