Dear Pallavi,
You can issue the letter as follows, if you find it matching to your requirement:
Mr./Ms. ____________________
Sub: In regard with Performance Deterioration and Usage of Office Resources.
This is to inform you that we are quite disappointed with your overall performance and Team Management Skills from the past few days. Please let us know in case of any help required from our end to help you improve your performance,Team Management Skills and usage of Official Email. In case we find your reasons proper and genuine, we would provide you with whatever help you require. Otherwise we would be forced to initiate a "Performance Improvement Plan" as part of which a specific set of standards of performance would be set for you to be performed within a set deadline. Also we expect every employee to understand that company provides Official Emails to use it for Official purpose alone not for personal usage. In case of your failure to match the set standards or prove your Team Management Skills and as well if you are found to be misusing the official emailing, the Management would be forced to take some serious action against your lack of expected performance, poor Team Management Skills and misuse of company resources.
You are hereby directed to respond in detail with your planned actions and agenda to this letter by (so and so date)
Thanks & Regards,
Ms./Mr. X