MailChimp will often ping Sugar with activities such as subscribes, bounces, and unsubscribes. If your SugarCRM instance is not publicly available outside your network this will cause MailChimp to not be able to keep your data up to date in Sugar.
To fix that, and keep your data as locked down as possible so that you don't have to completely open up SugarCRM to the world, we have created a nice proxy script. There are two simple steps to make it work:
1. Take this code and make it accessible from outside your network.
**NOTE: Be sure to change the $sugar_url variable in this script**
2. Add this script as a new webhook for each list that syncs to your CRM. Instruction on how to do that are here.