I have a nightmare story regarding this. I went on a two-week vacation and took a bunch of photos, and unknowingly had the "automatically sync on WiFi" option turned on. Well, at one point in time I did intentionally enable that, but forgot since I hadn't used the Google Photos app in over a year. Anyway, I get back from my vacation and realize I haven't received a personal email in the last 10 days. It took me a while to figure out Google Photos used up all my Drive storage, which then caused ALL my emails to silently bounce. No message or warning from Google that I had reached my storage limit, and of course there was no way to recover my lost emails.