concealed seizure of mail to or from suspects is permitted to investigate severe criminal offences (listed in § 100a sec. 2 StPO, Procedural law)
a clarification is needed, if content data is mentioned to be seized for the future or only saved content data
an open measure with notice to the suspected person is possible for saved content data in any investigation (§§ 94, 95 StPO) a delay of the notioce to the suspect is possible (§ 95a StPO).