How can I clear iCloud Mail storage?

Thanks for your reply.

Double check the value in the iCloud storage section. On an iPhone/iPad, go to Settings > [your name] > iCloud > Storage. On a Mac, go to System Settings (or System Preferences) > [your name] > iCloud > Manage.

You can also see the storage on iCloud.com by clicking the 2nd icon to the right (next to your personal icon) and choosing Your iCloud Storage.

i should have mentioned in my previous reply that the 3.4 GB you are seeing might be a calculation/display error. This is why checking the iCloud storage proper (see above) would provide a better understanding of.

Do you also remove the e-mails from the Trash folder (or Deleted Items / Deleted Message folder)?