stock email: activesync/exchange: selecting which folders to sync
solved while investigating to write a better question.

Apparently when the company migrated from some exchange version to another, something was set on the folders attributes that made android email application ignore it. Renaming the folder did not solve. Also comparing the attributes that are visible to the user between a folder that sync and one that does not sync reveals nothing.

Solution is to create a new folder, move the emails and updates the rules, and finally delete the old one.