How to move Mail storage to icloud drive?

Since iCloud Drive keeps a mirrored copy of all files on your Mac's hard drive you would not be saving any space on your HD by doing so. Any file added to the iCloud Drive will be mirrored in the Users/Home/Library/Mobile Documents folder.

iCloud Drive is a syncing feature, not an off-computer storage facility. This Apple document describes it in detail: iCloud Drive FAQ - Apple Support

You would need to create a sim link for that to work, but as Old Toad posted, a copy would still be on your hard drive. iCloud is a syncing system, not an external hard drive. If you moved it to iCloud, it would be taking up space in iCloud and on your hard drive.

Click the mail app on your Mac. On the menu bar, tap mail > accounts, tap iCloud and then uncheck mail.

You are welcome.

Open user library see this article macOS Sierra: Library folder

Find a folder mail , tap it to get highlighted , right click over it and click on copy mail .

Again , right click on the empty desktop and click on paste item.

The mails will start copying let the bar be loaded completely .

Open finder from the dock under iCloud Drive you can see the V6 folder , if you are using Mac OS Sierra the folder would be V4 .

But this process is done just save all mail data when you just want to keep the data on external hard drive or usb stick .

In your case if the mail data is excessive reduce the mail data by using the steps in these articles Free up storage space for email accounts in Mail on Mac - Apple Support

View, save or delete email attachments in Mail on Mac - Apple Support

Import or export mailboxes in Mail on Mac - Apple Support

Create or delete mailboxes in Mail on Mac - Apple Support

Read this also How to free up storage space on your Mac - Apple Support

What is 'Other' and 'Purgeable' in About This Mac? - Apple Support