Appealing redactions based on "interests of national defense"

I received a partial response to a State Dept FOIA request about an American found dead in a foreign country. One of the emails between Embassy staff was marked UNCLASSIFIED, but the email was redacted based on “National defense or foreign policy information properly classified pursuant an Executive Order. 5 U.S.C. § 552(b)(1).”

I can’t see any possible way an email that is marked UNCLASSIFIED unclassified and discussing what to tell the family of a deceased American would be classified pursuant to an Executive Order, but I’m not sure exactly what to say in my response. This is my first appeal and is due in a few days, so any suggestions would be very appreciated.

Thanks!