@bentalmage this should be turned on by default in your profile. Possibly this got turned off. If you sign in to your account, go to Profile in the upper right, then select Profile Settings. On the left side of the page, look for Invoice Settings>Invoice Preferences. This is where you can set the email notifications. See screenshot. If the box to receive emails when an invoice is submitted for your approval is checked and you are not receiving the emails, please let me know.
