Is the following requirement to the Can Spam Act (assuming that it is still a requirement) enforced:

'if the person knows or should have known that the recipient's address was obtained from an Internet website or proprietary online service that included a notice that the operator will not provide addresses for initiating unsolicited messages'

I understand that the FTC enforces the Can Spam Act. However, I cannot find this requirement anywhere on their site,

I know that there are several versions of this law on the Internet. Also, the government more than likely edits the law over time. Should I just go by what the FTC states as the requirements on their site? Thanks.