@hanky Why include the /mnt and all that?
Wine is able to access ext4 partitions just fine - though recall in a Windows program you will be using \ instead of /. And of course, your user will need read and write access to the files and folders.
I would have assigned K as /mnt/GENERAL/ and then set Opera's default mail folder as K:\mail\ or some such. It doesn't make sense to assign K as \ and then make root mail folder K:\mnt\GENERAL\mail\