Dear Landon Calhoun,
Good day! Thank you for posting to Microsoft Community.
In order to understand your query and provide possible solution, May I please double confirm if your client is using the new Outlook for windows (Preview) version or Classic Outlook with new build? could you please provide us with the screenshot of the affected outlook version details? see What version of Outlook do I have? - Microsoft Support
- If you are using the classic Outlook, may I please double confirm if the email in question is in your outbox folder? if you haven't check it yet, kindly check if the email is stored in your outbox folder, if you found there, kindly delete the email and check if there is any difference.
- If you are using classic Outlook, May I please double confirm if you tried to clear your outlook cache? if not, kindly clear your outlook cache and check if the issue still persists. To clear your outlook cache, kindly follow the steps below:
- If the Outlook app is open on your computer, you need to close it before you can delete the cache. Save any work you may be doing in it and quit the app.
- Press the Windows + R keys at the same time to open the Run box. Type the following path into the box and press Enter.
%localappdata%\Microsoft\Outlook - It’ll open a folder containing your Outlook profile and cache files. You want to find the folder that says RoamCache and double-click on it to open it.
- Select them all by pressing Ctrl + A on your keyboard, right-click on any one file, and select the Delete option.
- You may want to clear your Recycle Bin to ensure the files are gone for good.
- Then restart your Outlook to see the difference.
- If you are using classic Outlook, you may try to create new outlook profile and check if the issue persists, to create new outlook profile, see What version of Outlook do I have? - Microsoft Support
I look forward to your response. Kindly note that the above requested information is required for further analysis and troubleshooting of your situation. Your patience and cooperation are much appreciated.
Thank you for your precious time. Have a nice day.
Sincerely,
Libeamlak | Microsoft Community Moderator