So I suppose I should ask have you clicked on "Learn More" ?
I cant duplicate this because I don't have it.
My guess is that it will take you to iCloud Storage Management. See my example below.
You will see "Manage Storage" here, where you can decide what you want to store, Mail does not take up that much space.
It is likely that other storage such as Music or Photos is using the majority of your storage but obviously cleaning up your unopened mail will help a little.
Do not simply switch off storage for any of the major items like Photos if you are using iCloud Photo Library or you may loose many of your photos
If you have just the free default 20Gb of iCloud storage the obvious, easy solution would be to increase your storage to the next tier which is 50Gb for about $1.50/mnth depending on where you are.
My guess is that you are getting the notification in Mail because when your storage is full Apple will stop sending iCloud Email.
At the end of the day we all need to review our online storage plans. Apple would like us to store our photos, music and documents as well as the data that is shared by apps like Pages, Dropbox, Calendars, Contacts, Maps, etc across devices and 20Gb is woefully inadequate for that these days.
I get by very well on 50Gb and for the price it is well worth the convenience.
