I can't tell you how to check for spam in email, but StopForumSpam has an API that lets you can search by ip address, email address or username. This has stopped probably 99% of the spam I got before I started checking on that site. I see no reason why it won't work for emails as well as web sites.
A spam filter, at its basic level, is simply a set of rules applied to your data and it scores points based on those rules.
1. Does this address exist in the CBL = 1000 points
2. Does the subject contain "$1,000,000" = 20 points
3. Does the body contain "You have won the lottery!" = 50 points
4. Do I know this email address personally? = -20 points
Then you total up the scores and see if it is above your threshold.
To do this in C# would be pretty straightforward, string search and the like. The actual application integration might actually be more difficult.