I am expecting to send more than 500,000 emails this year. In this case, Emma Response Activities will occupy all my SF org data storage. Is there a way I can mass delete the old activities for certain less important mailings?
You could try using Salesforce's dataloader tool to mass delete out some of the old response data in their org which would clear up space. Though this is a pretty advanced feature of SF that we do not offer support on so if you have questions on how to implement it you'll need to contact SF support.