This is what you should include in a professional email.
Begin with a greeting:
If you know who you are sending the email to, it is always nice to begin the email with, "Dear Ms. Smith." Should you know personally know the recipient and are on a first name basis with them, you could use their first name. If you're not sure to whom the email should be sent to, begin with "To whom it may concern." Just remember that if this is a professional email, you should take some time in knowing who the email is going to. Beginning an email with, "Hey! I want you to publish my book," is not keeping it professional.
Start by thanking the recipient (if they are getting back to you.)
Let's say you are returning an email to someone who has queried you. No matter what the question might have been in the original email, it is always good to begin with, "Thank you for your recent email," or "Thank you for contacting us." People who are reaching out to a company want to know that they are appreciated. This is a great way to always be courteous, even if you're writing an email that isn't full of good news.
What is your purpose:
There is a reason you're emailing the recipient. What is it?
This might be the first communication you are having with the recipient, therefore thanks might not be called for. However, let them know why you are emailing by stating your purpose. "I am writing to enquire about..." or "in reference to..." Make your purpose clear before you get into the body of your test. Use good grammar, punctuation, and spelling. Get to the point and be clear about what you're trying to say or ask.
Make sure to thank the recipient for taking the time to read your email. This is a good time to also add a note of reply, if you're expecting one. "Thank you for your consideration. I look forward to talking to you." or "Thank you for your time."
How to end your professional email:
End with an appropriate closing. "Best regards," "Sincerely," and "Thank you," are all professional. Avoid "Best wishes," "See ya!" and "Cheers." Leave those to the casual email you're sending to your friend.
Of course, before you send the email proof it. Check your spelling. Make sure your contact information and your name is attached. Did you say what you needed to say? Do they know you're awaiting a reply? Always check your work.
One more point. If you're trying to make an impression of professionalism, check your email address. Does it say too much about you? If you're looking to be professional for the long haul, think about getting a professional email address that gives you that appearance as well.