As I often say, listen to as many sources of information as possible, and learn as much as you can from all of them. Don't be afraid to search for a second or third opinion on how to handle a
deliverability problem or list management issue.
To that end, it's worth checking out this page. This information from the makers of the popular SpamAssassin spam filter offers valuable insight into how their system works. If you’re a mail sender, it’ll help guide you on how to finesse your legitimate mail, to reduce the likelihood that it'll be incorrectly classified as spam.
I don't agree with every suggestion it makes (many non-spammy senders use open detection that sets off their "web bug" detector, for example), but overall, there's some good info here. Read it, learn from it, and see what easy steps you can take to comply with these guidelines.