Responses from emails not in my contact list

I just started using Constant Contact and so far so good, but there is a weird glitch that I can't figure out.

When I send a campaign, I get an autoresponse from a company that is not on my contact list.  I tried to use the chat help desk but they said it is likely someone's autoresponse to a forwarded email from one of my contacts. But this doesn't check, since the autoresponse would have gone to the person who forwarded the email. Also, the subject line doesn't include any FWD or RE designations.

Has anyone had this happen? If so, were you able to stop it from happening?