With a 17GB limit, Google actually lets you use up to 18GB of space before cutting you off -- stopping incoming mail from reaching you, and stopping outgoing mail from being saved to your sent mailbox. In theory, if I were using Gmail on the web, I would see a warning as I approached that hard limit, but since I never use Gmail on the web, I never see the warning.
Tuesday evening, I hit the limit, and of course the main clue is that mail stops arriving. This has been happening every few to several months, and each time it happens, I've deleted enough mail to get back under 17GB. (If I stay under 18, I'm fine, but once I hit 18, I have to get back under 17.)
I spent a little while doing this on Wednesday morning, but eventually decided to take the easy way out -- pay for more space. I hate paying for a "free service," but honestly, $1.99 per month or $19.99 per year isn't unreasonable to stop having to worry about how much storage I'm using. Adding 100GB to my capacity also means I can actively use my Google Drive, store images in Google Photos, and whatever else I've been avoiding for lack of capacity. (All of this shares the Google Drive capacity with the Gmail storage.)
Maybe I'll explore the other ways I can take advantage of that 100GB of additional capacity. I suspect it's now a total of 117, not just 100. At least, it should be!