Sorry if i am not posting this in the correct area..

My employer is tossing around the idea of sending out a mass email to potential customers. My question is how do spam filters determine what is or is not spam.
I have read that the emails header is reviewed and I have found a list of words that will raise the spam count. Is there any thing else aside from attachments i should be aware of? The only attachment will be a small image file (coupon).

Any advice would be great.