Hi Tilly03,
Thank you for using the Apple Support Communities! As noted in the article below "When you sign up for iCloud, you automatically get 5GB of free storage. You can use that storage for backups, Mail, iCloud Photos, iCloud Drive, and more. If you run out of iCloud storage, you won't be able to send or receive emails with your iCloud Mail address.":
Mailbox size and message sending limits in iCloud
We would also suggest the following article that can help manage iCloud storage:
Manage your iCloud storage
Hope this helps!
Have a great day.