How did the mail get hands on mehan's private lettter

I think there may be some confusion here!

I'm assuming you're referring to the famous letter written by Albert Einstein to his friend and fellow physicist, Max Planck, in 1915. The letter is often referred to as "Einstein's letter to Max Planck" or "Einstein's private letter".

As for how the letter ended up in the hands of the postal service, it's likely that Einstein simply mailed it to Planck, just like he would with any other letter. At the time, Einstein was working at the Swiss Patent Office in Bern, Switzerland, and Planck was in Berlin, Germany. Einstein would have likely written the letter on his office stationery, sealed it, and handed it over to the postal service for delivery.

It's worth noting that the letter was not intended for public consumption, and it's only because of its historical significance and the importance of the ideas discussed within that it has become widely known and studied today.

So, to summarize, there was no "Mehan" involved in the story of Einstein's private letter, and it's unlikely that the mail got "hands on" the letter in the sense that it was somehow stolen or intercepted.