What to do about a wall of spam?

Here’s a graph that shows traffic, over the last day, on our Internet connection here at the office (time runs backwards from the left-hand side of the graph):

See the two “mountains” of traffic, one yesterday afternoon and the other starting at 9:00 a.m. this morning? That’s all spam. Loads of it. Floods of it. Enough incoming network traffic that our SMTP server is having trouble keeping up.

In this case it’s not the spam itself that’s a problem — most of it is so obviously spam that we can easily throw it into /dev/null — but rather the impact on our bandwidth, and the server resources needed to identify and throw the spam away.

Looking at our mail server logs, this spam is coming from all over the place — there’s no discernable pattern of IP addresses or domain names that we could simply firewall out. And so I’m sort of as a loss as to how to react, other than to hope that, like the flood yesterday, this too shall pass.

Anyone have any advice to offer?

Update, the next morning: the spam flood seems to have passed, at least for now…