That post was from ELEVEN YEARS ago. Things changed a lot since then, most devices don’t even HAVE external SD cards nowadays. So no, you can’t change the storage location any more.
I have an internal SD card as do many other Android users. Most Android apps allow one to store their data on their internal SD card rather than on their buiilt-in storage. Why not K-9 Mail?
What do you mean “internal SD card”? If you can remove it then it’s an external one - the internal storage is called SDcard in most devices now since barely any OEMs still add the option to insert a removable SD card. I think only about 10 new devices still have them.
Google blocked that “apps to SD” stuff a long time ago already anyway, for reasons only they will know.