what is the cause of this? did they get in a spam filter? or simply deleted?
It's really not possible to say for sure. Some might have been filtered, some might have been deleted. Some might have been read as plain text and some might have been opened as HTML, but not had images loaded or clicked on any links so they weren't trackable http://www.campaignmonitor.com/help/topic.aspx?t=89.
From Australia to Zimbabwe, and everywhere in between, companies count on Campaign Monitor for email campaigns that drive real business results.Get started for free