Dear Kumar,
As per Indian Contract Act, 1872, appointment letter is contract letter between two parties. However, contract becomes legally enforceable when the other party acknowledges receipt of it. Therefore, on receipt of the mail containing the appointment letter, did you acknowledge it? If yes, then the contract is legally enforceable. Nevertheless, you may validate my proposition from the lawyer who handles cases on the contract act.
Your major defence is that you have just received email containing the appointment letter and not the hard copy of it. Since you did not sign the duplicate copy of the appointment letter, the terms mentioned in the appointment letter are not legally enforceable. Out of this assumption, you quit the employment. However, you did not create conditions that are conducive for your resignation. Therefore, your exit appears to be out of caprice.
You had a chance to raise objection for forcing you to work extra. Making you to work beyond nine hours per day is illegal. Therefore, on this count why you did not put application for restoring your working hours only for eight hours. If your company had refused to oblige you, it could have become ground for the resignation. Also you would have deprived your company to raise any objection against your resignation.
Anyway, this is afterthought. Now we cannot assess in what direction the things will go. Therefore, let us wait and watch.
Thanks,
Dinesh Divekar