In the invoicing system, there is a Sent History tab that shows which user sent this invoice to whom at what time. Here is how to use it:

After emailing an invoice, you will see a number show up next to Sent History.

Click on the Sent History tab, and you will see the log of which user has sent this invoice to him, as well as the time the invoice was sent. This helps your team to coordinate the communication with your clients.