If you are also using the QuickBooks Desktop Webmail feature, then the problem is very likely related to the incorrect webmail preferences. Your Email with the attached Invoice should be on its way, and a little pop-up message will confirm that QuickBooks sent it. Before diagnosing the error first, we have to find out why QuickBooks can't send emails using outlook and why does it freeze. The issues could be the QuickBooks outlook integration or with email settings.