A client called me yesterday scared and hysterical. He got a call from his hosting company. It seems the hosting company had been contacted by two major ISPs concerning spam emails they had traced to my clients domain. It seems that in the last 24 hours the tell-a-friend form had been used to send over 20,000 spam email messages! The titles of these emails where "Propecia buy Buy propecia Buy cheap Propecia online order cheap Propecia online. Propecia..." or "Take your poker game to the next level"
Do not place unsafe forms on your website that allows someone to directly email someone else. An alternative is one of the many social bookmarking scripts.
This is his one and only warning and they have threatening to yank his domain if they receive any more complaints. You can't blame them, they have their own reputation to maintain.
"Please take this warning as your first and last. Mail violations can place us on RBL's (realtime blackhole lists). RBL's are services designed to deny mail from outgoing SMTP servers that are regarded as spammers. Obviously this can severely affect thousands of clients sending regular, non-spam email. Unfortunately it then becomes preferable to disable the services of one client rather than risk having our e-mail services disabled for thousands."