The Can-SPAM act is in force in the US (other laws in other countries, but if your Email Service Provider is doing business in the US, and/or your email subscribers reside in the US, than your email marketing also must be CAN-SPAM compliant!)
Subject lines must clearly and conspicuously identify advertisement or solicitation. In other words, the subject line must be indicative of the actual content of your email.
A valid electronic return address working for at least 30 days
A postal address (P.O. box is OK) must appear – add a phone number as a best practice
Must provide an opt-out mechanism for subscribers. Instructions for opting out must be clear and conspicuous and honoured within 10 business days.
Don’t falsify or obscure the real sender.
Don’t “harvest” email addresses from the Internet.