You can control this.
QB will fill the Field from the email address in the data file; you can Hard Code anything as Text, and stop using a database Field that gets automatically filled for you.
Edit your templates and change from using the Field, to a Text Box, and fill it in. You can name your templates for the email address being hard coded, That means, if you and I both use POs or Invoices, we would each have a Duplicated Template, so they look alike, except for the email address. I would use my template, and you would use your template.