This is what I wrote to Dream Host support, and told me to post it here.
Hello nice people at DreamHost.
After browsing my spams, the spam mails are mostly coming from a couple of range of IP addresses.
I'd like you to add a feature to not receive any mails from specified IP addresses or ranges of addresses.
That would cut down good chunk of my spams.
A better feature is to parse the mail header and figure out the actual origin, but I would not ask that much for now.