I have been testing your plugin with the development version of the Mail Queues plugin and it seems to work well. Our (free) plugin let’s a wordpress site use multiple rate limited smtp accounts to get around send limits that may be imposed by their internet service providers. It works well for sites that aren’t ready for a commercial smtp sending service or who are very cost sensitive. It might be an option for some of your users who have the same concerns. If you try it and think we could should add any features to make integration easier for your users let us know.