Having gone through the CAN SPAM act for email marketing, need some further clarification. Their emails are in a public business directory where they have their full details along with emails.
Ironically, the CAN-SPAM Act was written so marketers CAN SPAM. There is no compliance requirement for opt-in nor distinction between B2B and B2C. You also don't need a label in the Subject Line (the federal law obviated the state requirements for this.)
That said, unless everyone in the public directory expects to receive spam, then sending to it is a terrible marketing practice and likely to get you blocked by ISPs and kicked off your email sending host. The two possible ways to approach this list are; 1) Pay the directory publisher to send a promotional email to this list with the advertising content, or 2) Send a 'permission pass' where the subject line and body copy are non-commercial, and simply requests their permission to send them something relevant to whatever directory they're listed in. This is ideally something non-commercial as a lead-generation vehicle like a white paper or event invite.
While email is the best and cheapest marketing vehicle ever, in a case like a directory mailing, you may find a better ROI with direct mail. It better accepted offline and since there's less of it now and more email spam--then it should provide an equal or better ROI for this type of campaignl.