Communication of acceptance is complete:

Section 4 of Indian Contract Act Communication when complete —The communication of a proposal is complete when it comes to the knowledge of the person to whom it is made. The communication of an acceptance is complete,— as against the proposer, when it is put in a course of transmission to him, so as to be out of the power of the acceptor; as against the acceptor, when it comes to the knowledge of the proposer. The communication of a revocation is complete,— as against the person who makes it, when it is put into a course of transmission to the person to whom it is made, so as to be out of the power of the person who makes it;  as against the person to whom it is made, when it comes to his knowledge.