When you send an email, its delivery depends on network connectivity and the availability of the recipient's mail system. Until the email is fully delivered, it will remain in the Outbox. If you lose your connection, BlueMail will automatically attempt to resend the email once the connection is restored. Once the email is successfully sent, it will move to the 'Sent' folder and be removed from the Outbox.
You can view your folders by swiping from the left edge or tapping the logo in the top left corner. Then, select the desired account and choose Outbox.