We created a new email campaign with multiple messages. We added the filter, Have Not Opened a Campaign Message, to our subsequent email messages. However, when calculating the audience on these subsequent messages, the filter is filtering everyone out and the audience is now 0. We haven't sent message #1 of the campaign yet, so how can this be?
This filter's definition, with key words highlighted, is "Select this filter to send this message only to members of the target audience who received, but never opened, a message from this campaign. "
If no emails from this campaign have actually been sent, then technically, no one has received this message and will filter out everyone by default. After you send the first email, you can recalculate the audience again to get a more accurate number.