Hello, everyone! This is Jackson Jon, a technical expert with 8 years of experience in resolving issues like 'Outlook emails stuck in outbox’. The possible reason for "Outlook emails stuck in outbox" maybe because your outgoing mail server name is incorrect or the outgoing server port number is incorrect. The other reason can be your email size is too big. If you are still facing this issue then connect with me any time for technical help. And I will be available at your service!