Really? So it's not my ISP?

Is your newsletter
originating from your Lunarpages site (with an LP "from" address), or are you sending from your home PC (via your ISP) with a "from" address of your LP site? If the former, it is (unfortunately) common for LP servers to be blacklisted for spam. Sometimes someone
is deliberately spamming (in violation of the Terms of Service), or someone's site has been hacked and taken over by a spammer, or someone is forwarding their LP email (including the spam) to another ISP, or someone's site is being used in forged return ("from") addresses by a spammer, or some idiot AOLuser pressed "report this email as spam" in order to unsubscribe from a newsletter, or...
In the latter case (
you are "forging" an LP return address onto your emails), the spam-detection software may be detecting that the "from" address doesn't match up with the actual sending IP address, and declaring it spam. You should investigate adding an "SPF" record (search Lunarforums for
SPF) to let them know that your LP site has authorized your PC to send mailings using its name (I
think that's the gist of it).
In either case, I don't think you can "pin" anything on your ISP. They wouldn't be marking your outbound emails as spam.