Turbine uses your email address as your unique ID on the system. It uses your unique email address to work out which Turbine account you want want to use. This means that you can't use the same email address twice, even for different Turbine accounts.
It also means that if you discharge an ex-employee on Turbine, you can't reuse that email address. This is because Turbine doesn't actually delete ex-employee accounts, it just puts them on ice in case you want to re-hire them and so their name is still attached to their requests.
However, this sometimes causes problems. For example, if you create a dummy user account for trial purposes, close it and then want to reuse that person's email address for a real live account.
To reuse an email address from a discharged user account, this is what you need to do: