How can I give a deleted user a new signup chance in multisite

I've started 2 multisite websites recently. With several of your plugins almost everything runs smooth. However, I have not been able to find the solution for 1 problem. When a user signs up, he enters his mailaddress, but when that particular user wants to remove the website he just activated he is no longer able to start a new website with the mail-address he already used. Even when I remove the old user on the Network Dashboard, the mailaddress still is an issue. Where and how can I remove this mailaddress so the old user can sign up again with his address?