I noticed that emails from people i opted in to that I usually just delete out my inbox eventually end up in spam automatically via yahoo mail. When I browse through my spam I see newsletters and all kinds of crap that I opted in for months/years ago some from dan kennedy and other top level IM's. I never un-sub'd from any of them because I was using them for headlines/email copywriting ideas. Yet they miraculously end up in spam folder every time.
Short version: if the open rate on inboxed emails is consistently low, then yahoo or who ever the email service provider is had there algorithm triggered to start auto flagging it as spam since the email owner never opens them only deletes them regardless of the fact that the recipient opt-ed in.
Solution: catchier headlines, more freebies, w/e you can do to get them to open more. If a majority of your emails are already hitting spam consistently, focus on generating more opt-ins and keeping them interested.