Where is the part that says this service is not going to use the obtained email addresses for spamming?
To be honest, I'm more comfortable giving out my phone number than my primary contact email address. On the other hand, it's much easier to set up a throwaway email than a throwaway phone number.
But it's to the users of the service to share their dialzoo url including their email. Like http://dialzoo/your@email.com
Our assumption is that people would use email that are consider public info already, and share that.
Perhaps this is more relevant for public profiles such as - business website - public social profiles like linkedin. - facebook - google +
Because in the latter case (which is what I think the service is), you could make the users register their email address (using a verification email to ensure they do not register someone else email without their consent) in your service and then have a map from a random hash to the email, allowing you to provide urls looking like dialzoo.com/nzef1824Ldaw68873 rather than forcing user to publish their email in clear. Or even require a username and use that for the URL.
http://www.developer.it/post/gravatars-why-publishing-your-e... http://archive.hack.lu/2013/dbongard_hacklu_2013.pdf