Group E-mail vs. Spam

Is there a difference between sending an e-mail to a group of people and spam?  Is the difference based on the number of people contacted?  If so, where is the dividing line between contacting a bunch of folks and sending spam?  Or is it something else?  Is it a matter of the intent of the sender?  If you are letting a bunch of people know about something is that different from electronic cold call selling?  Is it a function of the content of the message?  Does it matter whether you know all of the people on the list?  Is it a depersonalization issue?  What is the essential property of spam?