Jen Farhoumand the inactivity filters tend to confuse me a bit, especially when trying to think in double negatives. What I do is create a smart list of all the people that were delivered the Summer promo email, then in my new campaignTarget Audience say "Member of Smart List Not In - Summer Promo".
Another tip: in all of my campaigns, I add those who were delivered an email into a Static List. This helps if you're going to have a complicated smart list in the future, you can just say "not in list x". Then you don't have to wait forever for the smart list to load.
The link is now inactive as the community has upgraded - this one helped - Use Inactivity Filters in a Smart List - Marketo Docs - Product Docs - am looking for a Specific Description of inactivity filter