What's the difference between a television series and a television movie? The movie is over in 2 hours, of course, while the series carries on week after week.
Now what's the difference between an ordinary TV series and a soap opera? On a regular TV series, there is resolution by the end of each show. The viewer is satisfied that this particular story is complete. So why do they tune in the following week? Because they like the characters and the stories. This keeps them coming back for more.
But with a soap opera there is no completion at the end of each episode. Instead, the viewer is always left with one or more cliffhangers which sticks in their minds – what happens next? So the next day those same viewers are eagerly watching their soap opera again, because they're dying to know what happened. Problem is, every show is a cliffhanger, and so they get ADDICTED to the show.
Now imagine that soap opera is your email series – every day readers open their email to get resolution to something you wrote yesterday, such as the end to a story. But then you start a new story and you don't tell them the end until tomorrow...
You get the point. It doesn't have to be stories, it can be anything that makes the reader desperately want to know the piece of information you're withholding until the next email.
For example, “Tomorrow I'll tell you the 3 secret words to doubling conversion on every one of your salesletters.” Or, “Tomorrow I'll reveal the 5 common household products that, when combined and added to your soil, grow the biggest, sweetest tomatoes you've ever tasted.”
Do this every time and you'll see your email open rate skyrocket.