The article tries to answer the question about the admissibility of non-invasive control of diplomatic mail in the light of international law. Diplomatic and consular law has been codified in four UN conventions. Each of these documents contains provisions defining the status of diplomatic or consular mail and their means of transport. Attention is also drawn to one more important document prepared by the UN International Law Commission, namely the 1989 draft on the status of a diplomatic courier and diplomatic mail sent without the participation of a courier. However, this document has not yet taken on and will probably not take the form of an international agreement. The author tries to show that the formula adopted in the Vienna Convention of 1961 does not exclude the use of non-invasive means of controlling diplomatic mail. It also points to the controversial nature of Art. 28 of the UN KPM draft, which is one of the main reasons why this document did not go beyond the stage of the draft international agreement.