Below are some helpful hints about composing emails –
The “subject” should stand out. Be sure to include a meaningful subject line; this helps clarify what your message is about and may also help the recipient prioritize reading your email. Emails that contain “No Subject” can drive recipients—especially professors, teaching assistants, and employers in general—crazy!
Remember your audience. Just like a written letter, be sure to open your email with a greeting – e.g. “Hi Dr. Jones,” “Hello Ms. Smith,” “Mr. Mishrikey,” etc. After a few exchanges, it may not be necessary to greet your recipient.
Write like a pro, not like a bro. Use a standard font, and spell, punctuate, capitalize well, break thoughts into digestable paragraphs, and avoid abbreviations when you can. Further, there’s nothing worse than an email SCREAMING A MESSAGE IN ALL CAPS.
Write well and succinctly. Write clear, short paragraphs and be direct and to the point; professionals and academics alike see their email accounts as business. Don't write unnecessarily long emails or otherwise waste the recipient's time.
Be nice, but not too nice. Be friendly and cordial, but don't try to joke around (jokes and witty remarks may be inappropriate and, more commonly, may not be perceived as appropriate in email).
You can delete your Tweets, but you can’t delete your emails. Think twice about the content you’re sending. Think about whether the content is appropriate for virtual correspondence. Once you hit send, you cannot take back the message. If you do second-guess yourself, then maybe it wasn’t an appropriate email to send!
Remember, your teachers, teaching assistants, professors, and employers are reading what you write, and more likely than not, they will make value judgments about you through your writing.
End the email with your name.
 Opinions vary as to how lengthy an email should be. Some say it doesn’t matter and others say that an email should be as long as the text box without scrolling. Both perspectives have merit. In general, emails should be short and to the point.