The Outlook application in Office follows the same procedure while sending mails. But sometimes you may observe some peculiar behavior while sending mails with this component, such that:
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:
Thus the problem here is that until you open Send/Receive dialog manually, the email is not sent and remains in Outbox, indefinitely. Here is how you can get rid of this problem:
Outlook Email stuck in Outbox
Registry Disclaimer: Further steps will involve registry manipulation. Making mistakes while manipulating the registry could affect your system adversely. So be careful while editing registry entries and create a System Restore point first.
- Press Windows Key + R combination, type put regedit in Run dialog box and hit Enter to open the Registry Editor. 
- Navigate here: Substitute the placeholder x.0 with 15.0 for Outlook 2013, 14.0 for Outlook 2010, 12.0 for Outlook 2007, and 11.0 for Outlook 2003, and so on. 
- In the right pane of the above-shown window, look for the registry DWORD (REG_DWORD) named DelegateSentItemsStyle, it must be having its Data set to 1. Double click on the same DWORD to modify and get this: 
- Finally, in the above-shown box, set the Value data to 0 and click OK. Close the Registry Editor and reboot the machine to get this problem solved. Hope this helps you. Good luck! Related posts: