Secrecy of correspondence doesn't necessarily only apply to physical letters as far as Constitutions go. In Finnish constitution it is defined as "The secrecy of correspondence, telephony and other confidential communications is inviolable" meaning it also applies to any internet message.

Unfortunately large majority of parties in Finnish Parliament do not really care about that provision and have passed multiple laws which create exceptions to it. They do it via the proper protocol (which is essentially the same as modifying the Constitution itself) so it's technically legal.