It is true that before inflicting any kind of punishment on an employee, Domestic Enquiry has to be conducted. This is being technically correct.
However, in real life situations, when the misconduct committed by an employee is rather minor, he is given a Warning Letter without going through the process of conducting a Domestic Enquiry.
To answer your question, in such cases it is NOT mandatory to conduct a Domestic Enquiry before issuing a Warning Letter.
24th March 2012 From India, Mumbai