As per section 192A (3) of the Companies Act, 1956:
The notice shall be sent by registered post acknowledgment due, or by any other method as may be prescribed by the Central Government in this behalf, and shall include with the notice, a postage pre-paid envelope for facilitating the communication of the assent or dissent of the shareholder to the resolution within the said period.
For more details, read new Postal Ballot Rules, 2011 notified by MCA on May 30, 2011.