In some EU countries it is not only illegal, but also unconstitutional.
For example, Art 15 of the Italian constitution [1]:
"Freedom and confidentiality of correspondence and of every other form of
communication is inviolable.
Limitations may only be imposed by judicial decision stating the reasons and
in accordance with the guarantees provided by the law."
For example, Art 15 of the Italian constitution [1]:
"Freedom and confidentiality of correspondence and of every other form of communication is inviolable. Limitations may only be imposed by judicial decision stating the reasons and in accordance with the guarantees provided by the law."
[1] https://www.senato.it/documenti/repository/istituzione/costi...