Store partition is full
When I try to move iCloud mail on my iMac, MacBook Pro, iPad, or iPhone, I get an error about "Store partition is full". I have searched several other threads and see others have had the same issue, but the resolution is never the same. I have tried deleting my iCloud account from all four devices and reinstalling, to no avail. Hoping someone has some ideas here. Thanks!
iMac, OS X El Capitan (10.11.3), iMac (21.5-inch, Late 2013)