ployer wrote:My hotmail was doing that for some stupid reason as well.
One problem with services like hotmail, yahoo etc. is sometimes they can block the mail from a domain for everyone, based on the complaints of a few.
You'd be amazed how many people flag emails they asked for as spam. I work on another web project which has a signup, and afterwards members receive a confirmation email with all their account details and a activation link. Pretty standard practice of course and the email is very clear who it's from. Right after the launch we had something like 105 signups, and 16 people flagged the confirmation email as spam, even though they'd joined the site 2 minutes before hand.

Other peoples spam complaints can start sending your legitimate email into the filter.
We briefly had the problem here with hotmail (fixed now), probably the cause of your issue. It seems a people subscribed to topics for replies, and then marked the notifications they asked for a spam instead of unsubscribing from them. The link to change your notifications is right there in the email so I don't know... What can you do.