Sending a letter via post which will be legally valid

Sir,

You may send an inland letter per registered or speed post and track it on net. The receipt of letter and the contents are proved.

[inland letters are not to be enclosed in an envelope]

Good Luck

Kishor Mehta

The receipt would only give presumptions to despatch of letter but the contents can not be proved like that. You need to contact a lawyer and seek appropriate recourse.

you are perhaps not the first person who need to prove that a letter was sent.  You consult lawyer.