Maybe I'm missing something obvious but do how you get rate-limiting per
TCP *flow* with Cisco IOS ?
There is something called flow-based RED (FRED) but it consumes a whole
lot of memory because you have to keep track of lots more state. I
don't know about that code. At the least what you can do is use the
rate-limit command and rate limit *all* outbound TCP/80 traffic (or for
that matter all access-list captured traffic). Now, doing so will make
any but the most trivial outbound TCP/80 absolutely painful, and will
cause tail drop. See Cathy Wittbrodt's work in this space, which was
presented at NANOG some time ago.
Note, I'm not saying you should *do* this. It may be going a bit too
far for anti-spam.