There are several strategies that will help you compose this sort of email, some having to do with the form and some with the content.
First, you should send this to a distribution list set up as a blind carbon copy (BCC) so that the recipients don't end up seeing a list of 2,000 addresses in the "to:" field.
Next, your subject line should read: "New Employee Grading System Effective 2016". You should flag this email as important.
If you wish to have employees contact HR rather than yourself with questions, set the "reply to" field appropriately.
Your email should be no more than one screen in length. That means that you should include an active link to a page where the details of the policy are available rather than trying to include all the details in a single email. The general style of the email should be formal, including grammatically correct sentences and appropriate punctuation and capitalization.
The email might consist of three paragraphs:
- Reasons for changes and how the new system benefits employees
- Highlights of changes (perhaps as bullet points)
- Where to find more information