Proofreading

When sending emails to coworkers or clients it is very important to proofread your work.  Spelling errors can make you look unprofessional. 
Besides spelling errors it is also good to check to make sure you are using correct words, such as your, you're, there, their etc.  Make sure your sentences make sense and no words are missing or added by accident.
If it is an important email you may want to have someone take a look at it before sending.
It is also good to double check that your email does not send the wrong message.  It can be hard when reading an email to sense a persons tone.  Make sure to be clear about your message.
Once you've double checked and are confident it is all good, add your recipients and send away!