The problems with the Outbox can be related to the Send/Receive operation. The first thing you need to do is to manually force a Send/Receive operation. You can do this with the buttons available on the interface. The second thing you have to do is to check if the email you are trying to send doesn't have attachments larger than the maximum file size supported by the receiver's email service. Alternatively, check the server configuration in the Accounts section of the email client you use to make sure everything is configured correctly or create the email again and send it as this could be an error.