I have an iMac High Sierra 10.13.6. I also have an iPad and a new iPhone. I created several Mailboxesand initially saved them on my desk top but of course they didnt function on the pad or phone. I dragged them into iCloud and thought the problem
I have several mailboxes saved to my iMac and then created similar mailboxes on iCloud by dragging the boxes from the Mac folder to the iCloud folder but they don't sync and they don't link to my iPad or iPhone11
iCloud
iMac 21.5″, macOS 10.13