Fedora 26 comes with Postbooks 4.10.1 (Fedora 25 had 4.10.0) so you need to upgrade your Postbooks database when you move to F26.
But the Updater application errors out with missing libraries.
So the answer is to run it on Ubuntu 14.04 as a Virtual Box Guest
Other wise you will waste time trying to get libraries that are missing working:
Ubuntu 14.04 works flawlessly which is mentioned in the Compatibility Matrix