Sorry to hear you're having trouble receiving emails. If you still have around 20 GB of available iCloud storage, your account shouldn't normally be prevented from receiving emails because of a lack of overall iCloud storage.
I'd recommend trying the following steps on your iPhone 17:
1. Double-check your iCloud storage
Open Settings.
Tap your name at the top.
Select iCloud.
Check the storage bar at the top to confirm how much iCloud storage is actually available.
Tap Storage or Manage Account Storage to see what's using your space.
Please make sure you're checking iCloud storage rather than iPhone storage, as these are two separate things.
2. Check your iCloud Mail account
Go to Settings > your name > iCloud > Mail.
Make sure Use on this iPhone is switched on.
3. Check your Mail fetch settings
Open Settings.
Go to Apps > Mail > Mail Accounts > Fetch New Data.
Make sure Push is enabled.
Check that your iCloud account is set to Push and that Fetch is set to Automatically.
4. Check whether iCloud Mail is working online
This is an important step to help identify whether the problem is with your iPhone or your iCloud account itself.
Open Safari and visit https://www.icloud.com/mail
Sign in using your Apple Account.
Check whether new emails are appearing there.
If emails appear on iCloud.com but not in your Mail app, the issue is more likely related to your iPhone's Mail settings or synchronisation.
If emails aren't arriving there either, the issue may be related to your iCloud Mail account.
5. Restart your iPhone
Restart your iPhone and check whether new emails start coming through. Also, check that you're running the latest available iOS update under Settings > General > Software Update.
6. Check your iCloud storage and deleted emails
If your iCloud storage really is showing 20 GB available, you shouldn't need to purchase additional storage just to receive emails.
However, if the warning persists, check the Mail storage category under Manage Account Storage. You can also review your Trash and delete unwanted emails with large attachments if necessary.
I'd also recommend checking Apple's iCloud System Status page to make sure there isn't an ongoing Mail service issue:
Apple - Support - System Status
7. Contact Apple Support if the problem continues
If you've completed these steps and still cannot receive emails, I'd recommend contacting Apple Support so they can check whether there's an issue with your iCloud Mail account on their end.
You can reach them here: Contact - Official Apple Support