Three levels of understanding, maybe more.

I'm sure we all know someone who regularly forwards you the latest urban legend, Internet hoax etc. I have thought of three ways of understanding this, in ascending levels of complexity. As an added note, the senders all are college-educated professionals.

Level 1: They send these messages along because they honestly believe in the truth of what they are sending. This is possible because people seem to constantly over-estimate how much they know in areas in which they are not experts. For example, I have frequently worked with PhD life scientists who espoused economic ideas which would be laughed at as primitive in business school--in much the same way creationism would be treated by those scientists.

Level 2: They are well aware that what they are sending is unsound, but they are testing you. What they really want to know is whether you are smart enough to see through it too. So, in this scenario; when you send the snopes link debunking what they sent, you passed their "intelligence test".

Level 3: They know that what they are sending is a hoax AND they know that you will easily see that it is a hoax too. So, why send it? Maybe it will amuse you. Maybe they wonder how you will respond--will you think they believe in it and then act blunt or will you be diplomatic about it?