Yes, it is possible. According to the Article 218 of the CPC:
§ 1. Offices, institutions, and entities operating in the field of postal or telecommunications services, customs and tax offices, and transportation institutions and companies shall be obliged to provide to the court or to the public prosecutor, in accordance with the request contained in the decision, any correspondence and postal items (…) , where they are of importance for the proceedings being under way. Only the court or a public prosecutor shall have the right to open them or to order their opening.