If your are setting up email accounts for the non-technical it is important to simplify as much as possible. To this end lets disable the "On This Computer" folder in Evolution.

First make sure that Evolution is NOT running.

Then open up a terminal, and move to /home//.evolution/mail and then type the following:

rm -drf local
ln -s /dev/null local

Open Evolution, note that the "On This Computer" header does not have a "+" next to it and can not be opened.

Currently any email account has it's "Drafts" and "Sent" folders pointed to a non existent location. So...

Open Evolution Preferences, it's in the Edit menu, and for each of your accounts click "Edit", go to the Defaults tab and relocate the "Drafts" and "Sent" folder locations.