1. Not including the email thread in your reply.
Think about how many emails you receive every day. When you're communicating with dozens of people a day, sometimes you forget where you were in a particular conversation or what the conversation was even about, right? So it's nice to be able to skim through the previous emails to refresh yourself before responding. Do your recipients a favor and include the whole thread when responding. Although deleting the thread declutters the email and makes it appear less lengthy, in the end, it just creates confusion for the recipient.
2. Not using a professional account.
Syncing your professional account with your personal account is convenient. But when you have this feature set up, always double-check which account you are sending your mail from. Accidentally firing off a message from email@example.com will not only raise some eyebrows, it will mean the message will probably wind up directly in your recipient's Spam folder.
3. Not replying to all.
This one is so easy to forget. If the idea is to keep a number of people in the loop, then do exactly that and use the Reply All button. Enough said.
4. Cc'ing the world.
Yes, you need to reply to all, but before cc'ing someone into an email conversation, ask yourself if it's really necessary. Spare that person the gratuitous email if you can.
5. Forgetting the bcc field.
The bcc function is great for when you want to keep someone in the loop but it is not necessary for him or her to be part of the conversation. For example, after someone introduces you to a contact via email, move that person to the bcc field. Also, be mindful of people's privacy when sending out group emails to various recipients. Not everyone wants his or her email address exposed to a large group of people he or she doesn't know.
Emails should be short and to the point. If it's something you can't say in just a few sentences, or you find yourself in a nonstop, back-and-forth conversation, pick up the phone! Overly long emails may end up in the recipients' TL;DR (too long; didn't read) pile.
7. Writing unprofessionally.
Always keep a professional tone. That means ensuring emails have proper grammar and are free of slang. Save the "abbrevs" for Twitter.
8. Creating unnecessary back-and-forth.
When you're sending a quick email to set up a meeting, provide all necessary information in the first email. Otherwise, it becomes a back-and-forth conversation that could have been taken care of in one response from each side. For example, if you are requesting a meeting with someone, offer your availability. If you are scheduling a call, provide your phone number.