If there is a huge attachment to send, the sending process may get delayed or stopped at some point, resulting in outlook emails stuck in outbox Another explanation might be the installation of an add-in in Outlook. The mail server may not have authenticated the Outlook account. There might be issues with the mail server. If the application is being used with another software, the send/receive settings may be wrongly setup, or your Outlook emails may get stuck. Outgoing emails are analyzed by antivirus software, resulting in Outlook Emails Stuck in Outbox.