They could be real email addresses, unfortunately. Some of our clients have experienced situations whereby their email lists have grown with what look like legitimate email addresses, only to find out that on the next newsletter send, they get an excessive number of spam complaints and a shutdown of their email service. It seems weird that there are people out there who get off on creating malicious programmes that sign up random email addresses to lists. This is why the double opt-in exists and is encouraged so much by the likes of Mailchimp, as annoying as it might be to a merchant like yourself.
There are programmes you can use that can give you an indication of whether an email address on your list has been subjected to this type of situation. I've used https://www.datavalidation.com/ with clients previously to help pick out the email addresses that look like they might have been put added under bogus practices. Feel free to drop me a line if you want more info / advice on how to move forward.