Unless the user types it in. For now, I'll run through a short list of ideas for assembling your own e-mail marketing list. Notice that these are hardly original. Some are adapted for online marketing. Most can probably be worked successfully.
Web users are hypersensitive about commercial abuse.
Don't deviate from what you've promised. Just don't do it. It doesn't cost any more to keep it clean and honest. The short list above took me all of 2 minutes to put together. They're EASY to come up with.