Sign document by myself and sent it to the customer to sign it too

Hi there, Hi @AnandSri
I fill out my Acrobat pro DC document and sign it.
After I sign, the document must be read-only.

Now I want to send it to my customer that he has to sign the read-only document.

I set the option in my signature field as follows:
Read-only except the customer's signature field.

After sending, however, the customer can change all fields. Why?

can you help me.
Marc