"It looks like the newer version of the b2sdk package is installed and currently being used by mailinabox. If you run the command ls -lah /usr/local/lib/mailinabox/env/lib/python3.6/site-packages do you see b2sdk-1.16.0.dist-info directory listed?
If so i’d recommend deleting the b2sdk package files and reinstalling them using the pip3 command.
*stop the mailinabox service: sudo systemctl stop mailinabox
*delete the b2sdk python packages used by mailinabox: sudo rm -rfv /usr/local/lib/mailinabox/env/lib/python3.6/site-packages/b2sdk*
*Install the b2sdk package in mailinabox’s site-packages directory at the required version and upgrade any other packages: sudo pip3 install b2sdk==1.14.1 --target /usr/local/lib/mailinabox/env/lib/python3.6/site-packages --upgrade
*restart the mailinabox service: sudo systemctl restart mailinabox"