how to delete email messages on icloud storage

See the article Manage your iCloud storage - Apple Support

Delete messages and manage Mail

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.

iPhone, iPad, or iPod touch

If your iPhone, iPad, or iPod touch is set up to access iCloud email, you can delete messages from any mailbox, then empty the Trash to free up space:

1. Swipe left across any message to delete it.
2. Go to your Mailboxes and choose your Trash folder. 
3. To delete messages one at a time, swipe left on each message. To delete everything, tap Edit, tap Select All, and then tap Delete. Tap Delete All to confirm. 

Geminiman_25 Said:

"how to delete email messages on icloud storage"

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Deleting iCloud Mail:

Using iCloud Mail, delete the messages, then remove them from the Trash and Archived folders. Go Here: https://www.icloud.com/mail