Deleting a mail account on only ONE iCloud synced Mac I’d like to delete one of my mail accounts from one of my Macs, but not the other (due to the 15GB of email associated with it). When I go into Sys Prefs > Internet Accounts and hit the minus sign, the verification message says that it will be removed from all computers using iCloud Keychain and lists my other Mac (at least it warns me).  

I’ve gone into iCloud Drive and turned off the “Mail” entry, although it’s not clear what documents Mail stores there (perhaps AirDrop attachments?), but having that unchecked on both Macs makes no difference.

It’s nice the iCloud syncs internet accounts to you don’t have to re-enter them on your other Mac, but how do I turn off this one feature?

The Mac on which I’d like to keep the mail also has a 200GB iCloud Photo Library, so signing out of iCloud is a pain. But I’m not sure that would work. I would guess that when I sign back in, it would be minus the just deleted email account.

Running Sierra 10.12.6 on both computers.
The answer turns out to be the no, you can't delete an account on one iCloud synced Mac, without deleting it on the other(s). Beyond leaving said email account disabled, you must go into ~/Library/Mail/V5 in the Finder and uncover the multi-digit-letter folder associated with that account and trash the contents. That's how to reclaim the space occupied (in my case 15GB).