You could use NBAR.
Hope that helps.
if you want to check smtp traffic, you don't need any software. In the IOS there are some
filtering methods, so you can view only smtp flows... External software is necessary for continuos monitoring. I don't know any free widows version. There are several software with trial period for windows. You can check list of netflwo applications on our web site: http://netflow.caligare.com/applications.htm