Add the Recipient's Address Last
Sometimes when you're working on a somewhat lengthy email, you may accidentally hit the "Send" button, or hit the "Send" hotkey, firing off your email into cyberspace.
If the email was important enough, you may get that sinking feeling about poor working, incomplete thoughts, or quick notes that only you were supposed to see before you re-wrote them.
To avoid accidental sends, leave the To/Cc/Bcc field blank and leave it as the last thing you do before actually signing your name and sending it off.
If the address is one you're not familiar with or had just received, simply paste it at the top of the message so you can cut and paste it to one of the recipient fields when you're ready to send it.