Unfortunately the mail providers filtering does more harm than good in cases like these. Often times a forum email address gets blacklisted by email domains. This can be intentional. But often times it's simply mistaken behavior. All it takes is several members to mark the emails as spam to get black listed. Often times this is brought on by people not understanding thread subscriptions. Instead of clicking on the link to unsubscribe, they might mark the email as spam. This then causes the forum's email reputation on the mail host to be reduced, until enough people tip it into the spam world.
I currently have this problem with hotmail on my site.
I currently have this problem with hotmail on my site.