ip addy probably won't work for you, as most business browsers are browsing from an office where all users share a network external ip. So unless you want to limit trials to one user per department or building, that's probably a no-go.
I think I'd use that email addy, and send them something at that addy in the flow to confirm the trial, which would at least limit folks to a number of trials no greater than the number of email addys they actually receive.