Formula change value only once

Hi Friends,

I want the value of "Is Customer" column not change "email" column.

For instance, email column looks up in registration, and if that email exists in "registration" column, the value in "is customer" changes to "Yes". But once it changes to Yes I don't want to change it back to "No" even if the look up email is deleted.

E.g. The value for abc@gmail.com in "Email" should still be "Yes" even if the email's been deleted from "Registration" column.

I hope it makes sense.