If you are also QuickBooks Unable to Send Emails Due to Network Connection Failure 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.
Check the settings in the webmail preferences
On the left pane, choose to Send Forms. Under My Preferences, select the email account you are using, then select Edit. In the Edit Email, Info screen go to the SMTP Server Details section and set the server name and port to your email provider settings.
Regards,
James