A few months ago I gave Google Photos a trial. I wanted to give the service a real, honest test so I purchased one of Google's larger Drive plans (1 TB) and started uploading.
Google Photos is great. In fact, the only reason I didn't stick with it was because -- like so many Google things -- it had no desktop component. No Mac app. No way of managing or backing up my Library locally. So I've opted to use Apple's iCloud Photo Library and Photos apps.
During my short trial of Google Photos I had uploaded 30 or 40 GB worth of photos and videos. I then cancelled my Google Drive plan which would expire that extra space once that plan's expiration date came along. Once cancelled, my account would degrade back to what Google offers for free on Drive... a very generous 15 GB (especially compared to Apple's free offering of 5 GB).
Perhaps you see the problem coming down the tracks already. Well, I didn't. I didn't think that my email would stop working because my Google Drive account was overfull. Well, that is exactly what happened.
This would be OK if Google Drive would have let me know that my email would stop working in a for more clear way than it tried to. Here is the email I received when I cancelled my Google Drive plan:
This message confirms that your storage plan of 100 GB for firstname.lastname@example.org has been canceled. You'll be moved to the free storage levels for Google Drive, Gmail, and Google+ Photos on the day your subscription ends.
Aside: Pretty cute that Google still calls it Google+ Photos. Hanging on by their fingernails.
So I guess you could say I was warned. But not quite strongly enough I'd say. It doesn't say that I may want to free up some Drive space immediately so that Gmail would continue to function.
To make matters worse, once the subscription ended, it wasn't like Google could send me an email to let me know my email wasn't working. It wasn't until a day or two later that my sister sent me a SMS saying that my email was bouncing. I then had to log into Gmail to read the warning banner saying that I was out of space. I almost never use Gmail.com so I wouldn't have seen this. Also, Mail on macOS or iOS wasn't erring either. It probably should so at least I'd look into the issue.
I quickly logged into Google Photos, deleted a bunch of videos first, and email started trickling in. The email that trickled in was only from those with mail servers that were willing to continue to try to send the message through rather than giving up entirely. For any other email, it has been lost.
Live and learn. So I'm sharing this here so you don't have the same issue.