There are some exceptions of course: if you notify the employee and have a justified reason you can check some emails. So it's not outright forbidden. It matters a lot on the case and what you did, exactly.
I'm not aware of the situation in other countries.
(In Dutch: https://www.cnvvakmensen.nl/nextnow/blog/2016/march/mag-de-b... and https://www.sprengersadvocaten.nl/publicaties/wanneer-mag-ee...)