Note: This is a Chrome extension I built many years ago, but stopped maintaining. I built it for personal use, but it at some point had 14,000+ users.
Automatically Bcc any email address whenever you send an email in Gmail™
After you specify an email address to be Bcc'ed, this extension will automatically add that address to the Bcc box every time you compose (or forward, or reply to) an email in Gmail™.
This auto-Bcc extension provides a Bcc Me dropdown menu that appears at the top of the Gmail™ page. The dropdown menu (which can be hidden) allows you to quickly pause the extension's functionality or set other options.
This extension can be used in combination with Email to Salesforce, or Highrise's dropbox addresses.