There's a Parasite in my Gmail!!!

hello All!

I have email correspondence with a Local Government Department (a 'Utility' in U.S., a Council in U.K.). The emails I receive are altered AFTER I read them! I want to know how can the Sender modify his msges AFTER s/he has sent them?

I am not tech savvy but have the idea that if the Sender asks for a receipt he might be able to embed other information in the request. Is this possible?

I might add that the person responsible is a member of a Sovereign Citizen Group. Any Sovereign citizens out there who want to give me clue?

Thanks in Anticipation ...