When invoice emailed the to be emailed box gets unchecked

When I choose customer wants emails the check box is ticked, but when I email it from the invoice form, this does not cause the check box to uncheck like it does when I print it from the invoice form.  So quite often I will email invoices more than once (and after some have been paid which is embarressing) because they are still on the list to be emailed when they were emailed at the time of invcoicing.  I hope it is an easy fix as the 'to be printed' box unchecks when we print the invoice on the spot, so if a similar setting could apply to the email check box.  Thanks, Suzy