MarketingSherpa just released an article explaining what to do when the patent owners contact you about your email marketing program. Apparently a number of email marketeers have had to deal with this recently.
They've interviewed a bunch of patent lawyers to get practical advice. Plus, the article includes hotlinks to eight sites and blogs that may be helpful for you.
In short, MarketingSherpa's guidance is this: "If your company is contacted regarding a patent, it is unfortunately
going to cost you. At the very least, you must consult with a patent
attorney. (If you don’t, you will face higher damages if you are sued.)"