Hi,
Basically the issue is that the sent emails are stuck in Oubox, but can be sent when restarting Outlook.
We may try to reset the Send/Receive settings to check the result, Outlook saves the settings in a .srs file, we can rename the file to ".old", Outlook then will create a new one, check if the problem persists.
The location of the .srs file should be:
C:\Users\username\AppData\Roaming\Microsoft\Outlook
If the problem persists, we can also start Outlook in Safe Mode to test:
Press Win + R, type "outlook.exe /safe" in the Run dialog, press Enter.
Outlook will disable 3rd-party add-ins in Safe Mode, if the problem doesn't exist in Safe Mode, please consider to disable the suspicious add-ins to verify which one caused the issue.
Regards,
Melon Chen
TechNet Community Su