Change a field when a second e-mail has been sent in a Case

Hi,

I work with Cases in CRM,

When an email is sent to us, a Case is created with an Email which is OK.

I have a Field that is named "New E-mail" which is a Two Options Yes/No.

When the email comes in the first time, it is no which is OK.

But if a customer sends a second email on the same case, i would like to update the field "New E-mail" to Yes. So that we don't miss second emails on the Cases.

Any idea how to achieve this?

Thanks in advance.