Just think about it for a second. This person arrived on your website email list because they were interested in something you offer that is related to problem they are looking to solve. This automatically makes them a prospect for your offering, and therefore you should be interested in gaining the identity of that visitor. I know what you are thinking.
You already receive at least 50 emails a day from people and companies that are of no interest to you. With that and all the spam out there you spend too much time as it is trying to sort through and find the important email. I completely understand your thought process related to unsolicited email and am on your side 100%. I'd like to help shift your mindset a bit and show you how you can have your visitors volunteering their email address to you left and right.
There are a couple things you need to be aware of first though.... Your website visitors don't care about your products, your services, your company, or you. They only care about "what's in it for them." Although this may seem harsh and make you believe that people are completely self centered ... it's absolutely true. This can be powerful ammunition though, if you use it properly. If you truly want to build a massive email database you need to take the time to completely understand your prospective customers and find out what they want most.