GENERAL RULE
The basic rule that applies to direct marketing is that you need the consent of the individual to use their personal data for direct marketing purposes. As a minimum, an individual must be given a right to refuse such use of their personal data both at the time the data is collected (an "opt-out") and, in the case of direct marketing by electronic means, on every subsequent marketing message. The "opt-out" right must be free of charge.
You must also make clear who you are and where you obtained the individual's personal data (where this is not obvious).