You can view the email history of an Invoice through both the Web Portal and the App.

From the Web Portal > CRM Invoices > search invoice with filters to see the last emailed column


From the Web Portal > 
edit / select the Invoice > Scroll down to the History/Notes. This will show all manual and automated emails that have been sent relating to this Invoice as well as edit history. 

From within the App > accounts > invoices > select the invoice that you would like to view > history.