Recently at a place I worked for a spammer used a built in feature of our exchange server to redirect spam messages. Though it was a mistake with our server security, I do not blame the administrator, more MS and the spammer.

The long and the short is that our server and domain was flagged as a spam server and our normal email was denied by other servers. This caused us undue grief and caused our business dealings to slow down if not halt for a few days while we worked it out.

If I could find the spammer that did this I'm sure I'd give them more than a fine...