As suggested by Runal, this is wrong forum. Please put your question in right forum, so it will be addressed to right people.
Also, it would be easier for seniors to respond, if you give some more details.
1. Why the question of relieving letter? Has the employee resigned? OR do you wish to fire the employee?
2. Employee can send mail from company ID to personal ID. Important point is if the data was valuable / proprietory information of company?
3. Do you have any IT policies restricting this type of transfer of data? Do you have any HR policies in regards to support of this termination?
4. Have there been any communication with employee about the matter?
Best Regards,
Amod.