If not done so already click on settings then navigate to the last tab named "outbound emails". The 6 choices in the screenshot below are the methods of e-mail that can be customized when going through enter your hours. For this help topic we are using the "invoice to client" to demonstrate but the same principles can be applied to all e-mail templates.

After clicking invoice to client, the bottom of the screen will show you the e-mail and allow you to customize it to your preferences. Helpful information can be found in the right side of the screen under "available merge fields". These tags can be placed either inside of the subject line or into the content of a message itself. It is recommended that you use these tools to help personalize e-mails and provide crucial data such as invoice number, account number, etc.