You can issue the letter as follows if you find it matching your requirement:
Sub: In regard to 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 in the past few days. Please let us know if you require any help from our end to improve your performance, team management skills, and usage of official email. If your reasons are proper and genuine, we will provide you with the necessary assistance. Otherwise, we will be forced to initiate a "Performance Improvement Plan," which will include specific performance standards to be met within a set deadline.
Additionally, we expect every employee to understand that company-provided official emails should be used solely for official purposes and not for personal use. Failure to meet the set standards, demonstrate your team management skills, or if misuse of official emailing is identified, the management will be compelled to take serious action against your underperformance, 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 (specific date).
Thanks & Regards,
Ms./Mr. X