> Do you need an address to contact people on? You'll must make sure that the user can read the emails sent to that by you. Do you merely use it as a login-handle?

Pretty humongous dick move to use someone else's email address as one's own login for some website, wouldn't you agree? What if it's a popular website, and the owner of the address would like to use it for their id; why should anyone else be able to deprive them of that?

And thus it's also a dick move from the site operator to allow those dicks to do that. So no, it doesn't depend: Just don't accept untested email addresses for anything.