Write the actual message. Be sure to get your point across without rambling; if it's fluffed up, the reader may glance over the important details. Try to break up the message into paragraphs by topic to make your message more logical and digestible.
The email should be no more than 5 paragraphs long and each paragraph should be no more than 5 sentences long.
Insert a line break between each paragraph; indenting isn't necessary and will likely be lost during the email transfer anyway.
Be sure to avoid informal writing.