A federal law that regulates commercial email.
The law can be defined in four parts:
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It bans false or misleading header information
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It prohibits deceptive subject lines.
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It requires that your email give recipients an opt-out method.
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It requires that commercial email be identified as an advertisement and include the sender's valid physical postal address.
http://www.ftc.gov/tips-advice/business-center/guidance/can-spam-act-compliance-guide-business