I am assuming that your teacher has asked you to work on this as part of a professional writing class assignment. Although you can analyze and imitate the email above (copying it verbatim would be plagiarism and cause you to fail the assignment), rather than trying to intuit the structure of such an email from an example, it is easier to understand what elements you need to include.
Sender's address: You should invent a professional sounding email address. When sending company emails, you should always use your corporate account rather than a personal account. If you get to choose your email name, use a combination of your first and last name, e.g.
Subject line: Your subject line should clearly and concisely describe the topic of the email so that people don't just discard it unread. As many business people sort through hundreds of emails every day, this is quite inportant. You should not flag the email as urgent, as this is considered rude when done for simple and non-urgent administrative tasks (urgent is for "the building is on fire" not "please change your signature").
Addressee: For mass emails such as this, send a copy to yourself and then include the intended addressees in the "bcc" (blind carbon copy) field to avoid making readers scroll through a long screen full of names.
Body: Especially as many people now read email on their phones, the shorter the better. State in one sentence that all employees are required to change their signatures by a specific date, e.g. Monday 1 January 2021. Have a second sentence specify the reason for the change. Then, in bullet point form, list what should be included in the new signatures. Conclude by specifying penalties for non-compliance, apologizing for the inconvenience, and thanking people for making the change.
This is a good question. Here is a sample email.
To Whom It May Concern:
We are writing to inform all employees that the company will have a new policy regarding email signatures. The reason for this change of policy is to make the company more professional. Hence, the management requests that all employees by a set date change their email signatures to reflect the values of the company. In a subsequent email, we will offer a few sample. If you have any questions, please address them to your direct manager. I thank you in advance for your cooperation.
With this said, you might want to also consider using various forms of media to reach the employees, so that you can make sure that everyone is apprised of whatever you want them to know. So, you can write an email, verbally tell them at meetings, and send a physical memo.