Hi,
Did you deleted the mails or not able to located in the sent items ? Try the search option - https://support.microsoft.com/en-au/office/search-and-filter-email-3e32b06d-a2d9-4a66-922f-78b77c41b97f
Also depends on the archive policy if you have set you can recover the deleted items but if not it will be difficult.
Hope this helps.
JS
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