My ISP has recently moved me to having all my incoming mail come via the Postini (http://www.postini.com/) service. Since then a high proportion of the new posting notification emails received from the SMF forums has been tagged as spam. I've also seen a report from another user who has his own SMF forums and he is also finding that a lot of the messages are falsely identified as spam.
Does anyone have any experience of Postini specifically in relation to emails generated by SMF forums?
One main cause can be because your server is blacklisted as a potential spammer. You can check this by searching google for RBL checker.
If your a forum admin yourself, you can reduce the likelihood of getting spamblacklisted by disabling by default Auto-notification emails.
All the filter sees is hundreds of these auto notifification things come in every time someone posts. And it doesnt take a large proportion of users to hit the 'report spam' button to have it flagged.