Hey abubasim66,
We'd recommend reviewing the information in this article as it provides steps to help manage iCloud Mail storage: 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:
- Swipe left across any message to delete it.
- Go to your Mailboxes and choose your Trash folder.
- 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.
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. Then click Erase to confirm.
- Microsoft Outlook 2010 through Outlook 2016 (Windows): Delete the messages that you no longer want. Outlook will permanently delete your messages the next time you close and restart the app.
iCloud.com
- Sign in to iCloud.com.
- Select Mail and choose your Trash folder.
- To delete messages one at a time, select the message you want to delete, then select the Trash button
. To delete multiple messages, select the messages you want to delete then select Delete. "
If that's not working as expected, we'd recommend contacting Apple Support here: Apple Support
Thanks!