Your previous post screenshot was from the iCloud Backups section of Manage your iCloud storage - Apple Support
and that is what I was responding to. The following is from the Mail section of Manage your iCloud storage - Apple Support
You can free up iCloud space when you delete email messages from your iCloud email account. You can also move email messages from your iPhone, iPad, or iPod touch to your Mac or PC, where they no longer count against your iCloud storage.
Use the steps below to delete messages from your account. Remember that messages with large attachments take up more space than simple text emails.
and
Mac or Windows PC
If you use iCloud Mail with a desktop email app, such as Mail on your Mac or Microsoft Outlook on your PC, use these steps:
- Mail on your Mac: Delete the messages that you no longer want, then choose Mailbox > Erase Deleted Items and select your iCloud email account from the menu.