When I add a customer at Admin end, then deleted it.

When I add a customer at Admin end, then deleted it. after this, I use same email add a customer, it show error, this email was registered already. But I deleted it.

greatosale1 wrote:

It is by design. Nopcommerce does not hard delete the customers, products, and orders. The record is still at the database though not showing at the admin side.

You can set this setting to True
customersettings.suffixdeletedcustomers
and it will add 'DELETED' to end of email, so it won't appear as already used.
But, you need to have that setting true before you do the delete.
You can do it manually now if you have a SQL tool like SSMS.