There are few factors have been understnd before going to tackel your points:
Points are follows:-
1. Whether email resignation is acceptable by Court of law, either on an eletronic evidence or documentary evidence.
Ans:- As far as my knowldge the Hon'ble SC has been held several judgments that all email commuinications shall be valid in nature provided that email forwarded by concerned person and agreed.
2. Jurisidication factor:
Ans: Based on your employee resignation jurisidication or cause of action would not arise. Beacuse the appointment letter was issued by your company clearly would have stated about the jurisidication. Hence where your employees got his appoitnment letter in their jurisidication would arise or where he has served his last days services with the Company in that place jurisidication would arise.
3. Your Company Appointment Letter terms and conditions:
Ans: Please note that your company would have stated about jurisidication in the appointment letter as well as he can be transfer to anywhere. Please go through the terms and conditons of the appointment letter.
4. Whethere your Company can able to prove the same:
Ans: Most of the Comapnies will fall in this section, beacuse simply we cannot deprive one person fundmental rights and we cannot restirct indvuisal rights against their wishes. Or all terms will not binding on employees, if it is binding then it is against the ntaual justice of law.
So it is your Company onus to prove the same for loss or damages caused to the Company.
Better than the late just issue one formal HR notice first, then if he does not respnd then as a legal issue notice for recovery of monies for damages and loss.
Othern than this if your Company is really decided to file a case against him then it is really diffficult.