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To assist you better, I will need more information.
May I ask if you are using Exchange, IMAP or POP account?
Please also provide your Outlook version (you can check this in Outlook > About Outlook)
If you are using Exchange, please check in outlook.office.com, check if that specific email is in Sent folder or in Drafts folder?
If you are using IMAP, please do the same with your provider webmail.
If you switch to New Outlook (just for testing), does it behave the same as Legacy Outlook?
Have you tried remove and re-add account?
If yes and that doesn't work, try remove the entire profile. This removes the local database only; it does not delete mail from the server. But if you are using POP, make sure you back up the data first.
First, quit Outlook completely (Command +Q) > Go to Finder > in Go tab, choose Go to Folder and paste this ~/Library/Group Containers/UBF8T346G9.Office/Outlook.
Inside the Outlook folder, you will see Outlook 15 Profiles, double click it and delete Main Profile folder.
Now restart Outlook and add your accounts again.
Please try and provide more information so I can give clearer instructions.
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