When it was new, this law was nicknamed the U-CAN-SPAM act because it permits, rather than prohibits, unsolicited commercial email messages. As long as you provide the necessary verbiage and maintain the unsubscribe option, you can churn out all the junk email you want.
It's probably a good idea to avoid messages that combine commercial content with transaction or relationship notifications and information. That way, you don't have to make difficult decisions about coverage and your customers don't inadvertently opt out of receiving important information about the accounts they already maintain.
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...gone fishing.