OK you are right, Gmail may use IP-based rating, but it only does that if there isn't sufficient proof (in form of DKIM signatures) that the email is sent on behalf of the domain. If the email is DKIM aligned, then domain rating is used. I just didn't want to go that far into detail in my post.
> There is a reason Mailchimp owns a rather large block of IP addresses.
The reason is mostly for supporting smaller/legacy/self-hosted email services that do rely on IP reputation. Since Mailchimp won't allow customers to bulk email unless they have DKIM set up correctly, they don't have to worry as much about IP spreading with the major email service providers.
