K-9 Mail 5.600 only supports a couple of file managers. OI File Manager is one of them. But I believe on recent Android versions you won’t be able to save attachments on the external SD card this way.
Try the current beta version. It uses Android’s “new” mechanism to select a storage location when saving attachments. You don’t have to use any third-party file managers with it and it should work fine with your SD card.