The CAN_Spam act of 2003 is legislation dealing with mass emails. It states requirements for a company to include truthful contact information and clear headers in comercial and solicitation emails. It also includes rules for explicit content.
You have to be CANSPAM compliant. The easiest way to do this is just use a professional email marketing software. I use http://www.wirestamp.com it only cost $8 a month and is well worth it.