This email is written when you have definitely decided to decline an offer from an employer. It is an act of courtesy to notify employers as soon as you have arrived at your decision with a phone call, followed up with an email. It is not professional to leave a voice message declining an offer.
Dear Mr. or Ms. ____________:
First Paragraph: Thank the employer for the offer and be sure to state which position you were offered.
Second Paragraph: Include a statement such as, “I have decided to pursue other opportunities and will not be accepting your offer at this time.”
Third Paragraph: Close with an appropriate statement to keep the door open for possible future contact.