Why is my email full
How can I free up space in my email
Question marked as
You may delete eMails, or upgrade to more iCloud storage space.
Your account comes with 5GB, you can upgrade to 50GB in case you have important eMails that you don’t want to delete.
Storage plans are listed below:
https://support.apple.com/en-gb/HT201318
You can get 50GB a month for only $0.99!
Good luck.