text: Email Stuck in the Outbox in Outlook – Mailprotector Help Center
  Email Stuck in the Outbox in Outlook
  Messages sent from Outlook are stuck in the Outbox.
  Messages can get stuck in the Outbox of Outlook for several reasons. The most likely situation is due to a network connection problem or email account password problem. To begin troubleshooting, rule out that a network connection or password is not the cause. To assist with this first step, please follow the instructions in the Microsoft support article at https://support.microsoft.com/en-us/help/2663435/email-stuck-in-the-outbox-in-microsoft-outlook
  If the network connection and password are confirmed to be working, a potential reason for the problem exists if Outlook was upgraded with a different license type than the previous version. For example, upgrading a retail or OEM license of Outlook 2010 to an Office 365 license of Outlook 2016.
  Attempting an upgrade of Outlook to a different license type does not perform the process correctly; installing the new version of Outlook alongside the old version rather than completing an upgrade. This causes the Outlook versions to be in conflict, and the software behaves in unexpected ways.
  Installing or Upgrading Outlook with a Different License Type
  If you know the new version of Outlook is a different license type, uninstall the old version first. The settings and profiles should be preserved.
  BEST PRACTICE: Always perform a backup of the mailbox data and profile before attempting system or software changes.
  Once the old version is uninstalled, the new version of Outlook should install correctly and open the profile with the ability to transport sent messages from the Outbox to the mail server.