SpamAssassin is moreso for incoming SPAM. Not outbound...

Did the person from hotmail not send you the email message that was supposedly SPAM? If you received that you should be able to figure out who the SPAMmer is. If you did not get the original message along with headers and everything else, I would not worry about it. Although, doing all of the things listed above about closing an open relay should be done as it is the proper thing to do.