Ok so not truly a law but legal standards you need to abide by for the Can-Spam Act of 2003.  It was put into play to reduce the amount of spam and legitimize email marketing. While this law will not stop spam, it does make most spam illegal and ultimately less attractive to spammers.

The law has created requirements of how to send email and empowers the federal government to enforce the law. The penalties can include a fine and/or imprisonment for up to 5 years.

How you must change your email campaigns:

• List must be a permission-based list
• Email header information is not misleading
• Email’s “From” address is verified and accurately identifies you
• Include an opt-out feature of future email campaigns
• Email “Subject” line is straightforward, not misleading
• Unsubscribe requests must be honored within 10 days of the request
• Must include a physical address in your email campaigns