The CAN-SPAM Act spells out the rules for email marketers, whether their intentions are good or bad. If you’re sending emails for commercial use, you have to do the following:

• Don’t use false or misleading header information. Your “From,” “To,” “Reply-To,” and routing information – including the originating domain name and email address – must be accurate and identify the person or business who initiated the message.
• Don’t use deceptive subject lines. The subject line must accurately reflect the content of the message.
• Include an unsubscribe method. Your message must include a clear and conspicuous explanation of how the recipient can opt out of getting email from you in the future
• Timely response to unsubscribe requests. You must honor a recipient’s opt-out request within 10 business days. 
• If sending for commercial uses, you need to include a physical address. This can be your current street address, a post office box you’ve registered with the U.S. Postal Service, or a private mailbox
• While not explicitly banned, sending to people who have not opted in is discouraged