Outlook Email stuck in Outbox until you manually send it

When an email is sent in Microsoft Outlook, the email service first copies that email to the Outbox folder to check if the email can be sent. If a network error is encountered or the email is being sent to an address that doesn’t exist, the sending message remains in the Outbox folder. Otherwise, if everything is okay, the email is copied to the Sent items folder.

The Outlook application in Office follows the same mail-sending procedure. However, sometimes you may see some unusual behavior while sending mail with this component, such as:

You send a mail from the shared mailbox and it remains in the Outbox folder
When you manually open the Send/Receive operation window, an email is sent automatically
In some cases, you may first see errors when you open Send/Receive dialog:
In a few moments that paused email is sent:

The problem with this is that if you don’t open the Send/Receive dialog manually, your email will remain in the Outbox indefinitely. Here’s how you can get rid of this problem:

Outlook Email stuck in Outbox

Further steps will allow you to manipulate the registry. Creating mistakes while manipulating the registry could adversely affect your system. So be careful while editing registry entries and make sure you take a System Restore point first.

Press Windows Key + R combination, type put regedit in the Run dialog box, and hit Enter to open the Registry Editor.
Navigate here:

HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences

Replace x.0 with 15.0 for Outlook 2013, 14.0 for Outlook 2010, 12.0 for Outlook 2007, 11.0 for Outlook 2003, and so on. In the right pane of the window above, look for the registry DWORD (REG_DWORD) “DelegateSentItemsStyle” and ensure its data is set to 1.

Double click on the same DWORD to modify and get this: Finally, set the Value data to 0 in the box above, and click OK. Then, close the Registry Editor and reboot the machine to fix this issue.