iCloud storage question - how can I stop using iCloud for email storage and just store emails on my computer?
You must first copy or move messages, and then delete it.
How to move or delete email from iCloud
Moving your messages from iCloud to your computer can be useful when your iCloud mailbox is full or you want to reduce the amount of iCloud storage you're using.
Moving selected messages
Follow these steps to move messages from iCloud and store them on your computer instead.

In OS X Mail, choose Mailbox &gt; New Mailbox.
Select On My Mac from the Location menu, enter a name for the new mailbox, and click OK.
Select your iCloud inbox from the list of mailboxes in the sidebar, then select the messages to move.
Drag the messages to the new mailbox, or choose Message &gt; Move To &gt; the new mailbox.

Deleting selected messages
Follow these steps to permanently delete messages from iCloud without keeping a copy on your computer.

Select the iCloud messages to delete, then choose Edit &gt; Delete. The messages are moved to your Trash folder in iCloud.
Choose Mailbox &gt; Erase Deleted Items to permanently delete the messages in the Trash folder.

How to copy email from iCloud
Copying your messages from iCloud to your computer can be useful when you want to have a backup.
Copying selected messages
Follow these steps to create a copy of your iCloud messages on your computer.

In OS X Mail, choose Mailbox &gt; New Mailbox.
Select On My Mac from the Location menu, enter a name for the new mailbox, and click OK.
Select your iCloud inbox from the list of mailboxes in the sidebar, then select the messages to copy.
Drag the messages to the new mailbox, or choose Message &gt; Copy To &gt; the new mailbox.

Copying the entire mailbox
Follow these steps create a single mbox archive of your iCloud mailbox, including every message and folder in the mailbox.

Select your iCloud inbox from the list of mailboxes in the sidebar.
Choose Mailbox &gt; Export Mailbox.
Choose a destination folder for the mbox archive, then click Choose to save the file.

iCloud: Using OS X Mail to move, delete, or copy iCloud email