Try running ``mailq'' (or its equivalent, ``sendmail -bp'', if mailq doesn't exist for some reason) and see if you see your emails sitting in sendmail's queue. If you do, then you need to fix sendmail. If not, check root's mail and the mail of whatever user is running Formmail (should be the same user that's running your web server) and see if you can see any bounces. If so, then you still need to fix sendmail. If neither of these are true, then you probably need to fix Formmail.