I have just been told that my recent bout of spam from a plethora of different organizations (impossible to block by address) has a common source: a database company named Cision.
Do not deal with Cision. They sell addresses harvested by unethical means, and people will get angry with YOUR organization if they start getting unwanted mail because you bought one of their dirty mailing lists.
Thanks to the American Chemical Society, who divulged the identity of the malefactor.