Are there any Plans to integrate NIS Spamfilter into Thunderbird?
Currently NIS Spam Filter will work only with Outlook, Outlook Express and Windows Mail. I am not sure about the future release of NIS (NIS 2010), the latest Beta- Norton 360 3.0 also follow the same and Antispam don't support Thunderbird. But email scanning is supported on Thinderbird. Norton AntiSpam scans incoming email for spam with any other email client that utilizes the POP3 protocol for email. It does not integrate with these programs, and as such, will not be visible in any menus or toolbars within these programs. Thunderbird is much more advanced than Outlook/Outlook express and has it's on Spam Filtering as built-in. I think, that's the reason why there is no intergration for Norton AntiSpam.
"Thunderbird is not an officially supported email program for Norton AntiSpam. While it is supported at the protocol level, you won't be able to get any of the toolbars to integrate."-taken from a post by Tony_Weiss