When writing an email message, make filling in the ‘To’ and ‘CC’ fields your final step. That way, you’ll always have a chance to proofread and edit the message without any chance of accidentally sending it out before it’s ready.
Most email programs automatically drop the cursor into the ‘To’ field when you open a new message. As a result, we’ve been trained to start by filling out the email addresses of our recipients before we even type the subject line, let alone the actual body of the message. If I were designing email software, I’d set the default so the cursor dropped into the body of the message first.
But until that happens, my advice is that when you open a new message, your first step should be to get your cursor out of the ‘To’ line, drop it into the message body, and start writing.