I'd recommend contacting Hotmail. Today, lots of valid mail trips filters - ecommerce order confirmations & invoices, forum post notifications, forum registration confirmations, legitimate newsletters, etc.
It's possible that a few of your members flagged a past mailing as "junk". It's easy to do by mistake, and some people are just lazy. They'll block an email rather than clicking on the "unsubscribe" link.
I haven't seen a statistic for a few months, but a while ago a study showed that only 90% of legitimate email reached the intended recipient's inbox. This is the most insidious effect of spam - the time spent deleting is a pain, but the corruption of email as a reliable way to send important information is a true crisis.