Oops. You said "an email." I suggest that you write the email EXACTLY in the same form as if it were a written letter, and follow up IMMEDIATELY with a mailed, paper, written letter.

The important thing is to write, and to include the words "Thank you." If you haven't formally accepted the job in writing already, then you need to say something saying you accept the offer. Include the date. Include all your contact information in case they need to reach you, even if you think they have it already. It should be short. You are not communicating anything factual, the content of the message is just "Thank you." The tone should be normal "business English," not colloquial. It might be appropriate to mention the date of the offer letter and the starting date. The letter should be addressed to whomever sent you the offer letter. Echo whatever style they use in their letter to you, except when in doubt be more formal. If they close "Sincerely yours," close with "Sincerely yours." If they close with "Very sincerely yours," close with "Very sincerely yours." Daniel Smith 1234 Main Street Zenith, Winnemac 54321 [Phone number] [email address] July 23rd, 2015 Dear Ellen Sullivan Thank you very much for your job offer of July 21st, 2015. I accept with pleasure and look forward to starting work at Megacorp Widgets International on July 30th. Very sincerely yours Daniel Smith

If you have a specific person to email. Just say something like. Hi (your name), Thank you for selecting me for the position! I am very excited to start working for the company. Thanks, (your name) You can say whatever you want, it doesn't need to be long, as you've already been chosen, and you don't even need to say anything in particular.