Hi Anita,
Please.
Hope you have the good internet connectivity please check to make sure that first.
There are few options that you could try quickly and see how it behaves...
By Starting Outlook in Safe Mode, you’ll prevent the message from being locked by an add-in like for instance a virus scanner which integrates itself with Outlook
Please try the following methods as described in this article
https://www.howto-outlook.com/faq/messagestuckinoutbox.htm
Hope this helps!
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