In that case you have an option to delete the email after you have sent it on Gmail. Here’s a step by step tutorial on how to do do.

Learn how to remove auto-saved autocomplete email address suggestions from the To field in Gmail. Stop Gmail from remembering email addresses using this tutorial.

I recently discovered a little hack that can help you to bulk delete all the messages in your inbox at once. I know that this isn't a new trick -- there are tutorials on YouTube over a year old.