What CAN-SPAM requires
The FTC compliance guide lists the main requirements: no false or misleading header information, no deceptive subject lines, identify the message as an ad, tell recipients where you are located, tell them how to opt out, honor opt-outs within 10 business days, and monitor what others do on your behalf.
The law makes no exception for business-to-business email, and each separate email in violation can carry a penalty of up to $53,088, according to the FTC guide.

