This indicator shows the number of participants who have never received an email from the project.
Both manual and automated Communications count.
Depending on your process, this may or may not be a problem. If participants are meant to access the course through an invitation email, the indicator will remind them that the necessary precondition has not been met yet.
Notice that the indicator cannot interpret the content of an email sent from the project. Therefore, it will consider any email, regardless of whether it is an invitation email or not.