group domains for the purpose of domain throttling
This plugin is a community effort to identify domains that belong together, so that they can be grouped for the purpose of throttling.
Example
For example, if you are sending to Yahoo.com and Yahoo.co.uk then it will be useful to group these two domains for counting. That will help to prevent sending too many emails too quickly to Yahoo, which will help to ensure delivery to Yahoo.
Please send PRs for this plugin to increase the current list of domains.