This reaction is rare with an email autoresponder course. Even though they’ll be added to your general email list, they still expect to get emails from you and are more receptive to all of the email you send. They get used to seeing your emails come in through your email autoresponder course; your other emails arriving to their inbox are no big deal.
Search terms: You can find out the specific terms people are using to find your site, which will help you decide what the most popular topics for your readers are. However, Google searches are increasingly not providing the specific search terms that they used to, going with “not provided” instead, so this ability may decrease in the future (at least with the Google search engine).
Customer reviews are the "social proof" that encourages people to join in on something. It's one thing for you to tell people to sign up for a campaign, but it's another thing for your happiest customers to say it too. Publish your best reviews from communities like Yelp right to your website. This adds genuine value to your landing pages when people are on the fence about submitting their contact information.
In this context, I guess each post can’t be thought of in isolation, but in terms of a mini content ecosystem that comprises of a great blog post optimised for on-page SEO, an upgrade to that post tailored for that specific content, various on-page email conversion points including an exit pop-up or similar -and then you combine all this with your off page link building and outreach efforts. That could be over 5,000 words of content all in just that one package.