Al Iverson wrote an excellent post on business contact databases and opt-in. Here's a summary:
For email addresses retrieved from services such as Zoominfo and Jigsaw, do opt-in requirements and CAN-SPAM requirements still apply?
Contacts found on sites like Jigsaw and Zoominfo (and similar services) haven't opted-in to receive emails from you. If you take email addresses obtained from a service like this, and you add them to your list, they're going to report your mail as spam in very high numbers. It's going to get you blocked at ISPs. It's going to cause blacklisting issues.
CAN-SPAM (and opt-in permission requirements) still apply in the B2B realm. Keep in mind that B2B filters like Postini, Barracuda, MessageLabs, and others, they all work in a manner very similar to how ISPs work. They receive spam reports from unhappy recipients, people forwarding spam, or people clicking on a "report spam" button in an Outlook plug-in. They look at reputation measures in much the same way that ISPs do. And they will block you for spamming, just like the ISPs will do.
In the B2B world, it can be harsher on you when this happens. If you're blocked at Yahoo, you know specifically that you're blocked at Yahoo. But, if you're blocked by Postini, you're blocked by the thousands of companies that use Postini as their spam filter. It becomes a much broader issue, one that can be a lot more difficult to investigate and resolve.
The moral of the story is, when you're getting a person's email address from somebody other than the user of that email address, then you shouldn't be emailing them. You don't have permission from the user of that email address, and if you add them to a list, and send them email, you're spamming.
Contact databases are a useful tool, but not for email list building.