What constitutes a non-responder in reporting?

A non-responder is a recipient who was not tracked with an open, click, an unsubscribe or a bounce for a sent email.
 
Please note that an open can only be tracked for HTML email and is dependent on images rendering for the recipient. it is possible that a recipient opened an email but either received a text-only version or has images blocked by their email client. In these cases if they neither click a link nor unsubscribe they will also be counted among the non-responders in reporting.