how to set up collaborative spam prevention?

server side spam protection for big mailservices

[muesli]

i operate a website that includes free email for all users (no attachments allowed), a million active users, web-based mail-client.

the problem: the current server-side spam-checks hardly have any effect, we get washed away by spam.

i use a collaborative spam-tool on my own outlook-mailbox (cloudmark spamnet) and it does a great job. therefor i'd like to do the same on my website. as i have enough users to build up a collaborative spam-database myself, i wouldn't have to invest in supposedly very expensive access to spam-check networks.

i.e. i'd like to display a button "mark as spam" at every message-view-page and use the collected data to move spam to a user-controlled spam-folder automatically.

does anybody have experience in building up collaborative spam-protection? any suggestions for open source software?

[DaveAtIFG]

Ummm... What server software do you need to run it on? ;)

[muesli]

we use postfix as mailservers running on linux machines but we could probably implement virtually any technology as most parts of our messaging systems were "handcrafted" inhouse.