If you have tried all the suggestions below and none have worked, try this:

In Microsoft Outlook go to File > Options > Mail. Now, this will only work if you have set your default email.

Under Send messages > Click the box next to "Always use the default account when composing new messages"

If you have not set your default email account in Outlook, then do this:

In Outlook click on File > Account Settings > from the drop down menu choose Account Settings > under the Email tab, click the email you want to use > Click Set as Default

That should work.