When you setup a Google account, you agree to Ts&Cs that mean, you give Google a license to do pretty much as they wish with any of your content, including email if you use Gmail.
I don't have a Gmail account and, never sign-in to any Google services.
The following from Google:
"your content
Things that you create, upload, submit, store, send, receive, or share using our services, such as:
- Docs, Sheets, and Slides you create
- blog posts you upload through Blogger
- reviews you submit through Maps
- videos you store in Drive
- emails you send and receive through Gmail (Emphasis mine)
- pictures you share with friends through Photos
- travel itineraries that you share with Google"
policies.google.com