I have this exact same problem described in this thread.  When I installed Quickbooks I was using a different (personal) default e-mail in outlook than I want to use now.  I have now changed Outlook to the correct (company) default e-mail.  If I generate a new email from within outlook it chooses this new (company) e-mail address by default without problems.  If I send an invoice from within Quickbooks is still using the old (personal) default outlook e-mail not the new (company) one.  There appears to be no way of changing the default email address Quickbooks uses.  It is set permanently at the time Quickbooks was installed.  There is no way to access the default Outlook e-mail account in quickbooks to edit or delete it.  Quickbooks overwrites or changes the current Outlook default account to the one that was default at the time it was installed.  It's the same for Estimates and Purchase orders.  None use the current default account in Outlook they all use the one that was default at the time of installation.  Very very frustrating.