Used stamps exchanges.

https://www.imgup.cz/images/2020/01/14/01c.jpg

Sample official stupidyty, mail letter returned back to the sender.

International mail is rule by UPU regulations and unless there's some kind of censorship by a particular country the mail should be delivered to the addressee if deemed safe after inspection.

According to the laws of the Czech Republic, they had no reason not to let him go. When returning the sender must indicate why.

Согласно законам Чешской Республики, у них не было причин отпускать его. При возврате отправителю необходимо указать, почему.

I think the customs did not permit the letter. So, you were lucky to receive it back. In Russia, I think, it could be thrown away by custom servants.

There was no explanation. The letter was just open, stamps, I did not count, but he did not seem to miss anything. Sealed was a customs official TAB. The tab overlaid the address. Therefore, the mail returned the letter to the sender.

What is the reason of this letter returning (some paper with explanations or some mark on the envelope)?