You didn't say anything about sending an email. You didn't mention if you were told not to send the email, if you sent it from someone else's computer, whether the content was offensive or, indeed, contained blatent insubordination.
Have a read of this:
The concept of insubordination is most often associated with military organizations as military organizations have a chain of command and lawful orders given by a superior officer are expected to be carried out by the person to whom the order is given. However other strongly hierarchical organizations use insubordination as a reason for dismissal of an employee.
The penalty for insubordination in military organizations can range from dishonorable discharge, incarceration for a term of years, to capital punishment. The penalty varies depending on the type of infraction, the circumstances, and the culture of the military force. In modern Western armed forces, the penalty tends to be dishonourable discharge and/or a prison term. In other circumstances such as the German or Russian armies on the Eastern Front during World War II, the penalty tended to be an immediate execution. Selim I killed two army officers after he took power charging them with insubordination.