The President of India, according to the Constitution, must address their resignation letter to a specific constitutional office holder.
Article 56(1)(b) of the Constitution of India outlines the grounds for the removal of the President, which includes resignation. The procedure requires the President to submit their resignation in writing.
The President of India submits their resignation letter to the Vice-President of India.
Upon receiving the resignation, the Vice-President is required to inform the Speaker of the Lok Sabha.
Therefore, the President of India addresses their resignation letter to the Vice-President of India.