Although this is somewhat an older error we do still have some of our long time clients who will get their error because we’ve left legacy services in place not to disrupt folks.
When you receive this error, it more than likely means that you will need to cycle though a send receive in your Email Client. To Fix this, its as simple as hitting your send and receive button and then attempt to send your email again.
To avoid this error you can go to Tools Preferences and change the order your mail clients works in. By default it does send receive. By changing the default setting, you can make it receive first and then send or you can change to using SMTP Authentication and check the box that says my outgoing server requires authentication.
In any of these cases, it will correct the issue. If you need further support please email firstname.lastname@example.org and we’ll be sure to get someone back with you.