Sir/s,
"I mean proof of letter sent as well as proof of what was written in the letter."
As I understand the query you need to prove the contents of the letter sent per Registered or Speed Post: You have only to prove that a certain letter has been posted at a given address, and if the address is correct it is assumed that the letter has been received at the destination, however if the receiver wants to contest that the said letter is not received then he has to produce the letter that has been received in the envelope and prove that it bears the signature of the sender and has been sent by the sender.
Good Luck,
Kishor Mehta