You can also create a special opt-out. At the bottom of the email, just above the unsubscribe link, you can put something like “I’m not interested in this calendar”. When they click it, tag them, something like “Never send the calendar”. Then you filter out those contacts when sending the calendar each month. Be sure to remove the “Never send the calendar” tag on any opt-ins that you create, or you’ll get conflicting automation, which defeats the purpose.
You can also create what I call an interest tag. When they click to download the calendar, they are at least slightly interested. They at least want to look at it. So if they click to download the calendar, add a tag, something like “Calendar interest”. You can use this however you like. I more or less use it as a simple pointer, sometimes filtering email broadcasts based on it. One thing I’ve considered, but not implemented, is that if it’s been a while since the tag was applied, remove it. This would indicate the customer’s disengagement with that particular interest.
The point of the interests tags is to create lots of them; basically everything they can click in an email should apply a tag. After some time of people clicking, you can get some data on what your customers actually want. This will give you ideas to move forward with new projects, or drop the loser projects. If you find particular interests are always well received (e.g. a big portion of your customers have the Calendar interest tag), you can go for the “special opt-in” and ask them explicitly if they want this thing. Then just add a tag, like “Calendar opt-in”. You can then use this exact same opt-in for gaining new customers. If your existing customers are opting in to the Calendar, then prospective customers likely will as well. You can move ahead with confidence and lead generation on your Calendar project because you will have data behind it.