I use Outlook 2013 for several @gmail addresses and an Exchange account that connects over the internet. To force the selection of a sending account whenever I click the "New Email" button, this registry entry works well:
Value Name: ForceAccountSelection
Value type: REG_DWORD: 1 for force or 0 to disable
Another way (as many people know) to create a new email is to click on someone's email address in an existing email. It's nice that it fills the To: field with the clicked-on address, but it also ignores the ForceAccountSelection registry key. Instead it goes ahead and sets the From: to my default @gmail.com account.
Lately I've forgotten to check before sending and have sent from the wrong account. How can I force Outlook to always require the selection of a From: account? Thanks.