It was based on IP address.

You're right in that all hosting services suffer this problem to some degree, but it's a particular problem on EC2 since it's so easy to request IP for an instance that does something bad, then release back into the pool for other instances to use, a short time later.

It only costs a few dollars to that on EC2, whereas most other hosting services require at least a month's commitment and higher fee(s) before you can defile one of their IP addresses.

BTW, the reason I posted was to find out what other blacklists might be out there, so if your company is blocking access, I'd appreciate letting me know which service they're using.