Giveaway campaigns are a very popular marketing tool, and when run properly and adhere to state and federal regulations, are fully legal. Every entrant into a giveaway opts-in to a set of Official Rules which should state that they are agreeing to have their email shared amongst the sponsors of the giveaway as a condition of entering to win.
You should always clearly state the terms of the giveaway in your Official Rules and make sure you are familiar with the language. Rules generally contain the fact that there is no purchase necessary to win; the starting and closing dates of the contest; details of entry procedures; winner selection and notification procedures; a list of prize(s); privacy considerations for information gathered and eligibility requirements.
We offer an Official Rules Generator which you can amend to your needs, but note we can't be held liable in the unlikely event a dispute arises as a result. If you have further questions about your brand's specific needs, consult your legal counsel.