Hi @Darren Rose ,
Do you know the email address of each sender? If you know it, you can create an eDiscvoery case in Microsoft 365 compliance to search for a specific Office 365 mailbox based on the sender’s email address as the search criteria. But it should be noted that to use this feature, you need to have the appropriate license and be assigned the eDiscovery permission.
Regarding how to use Core eDiscovery and how to search for content in Core eDiscovery cases, Microsoft has given a detailed step-by-step introduction. You can refer to the following articles: Get started with Core eDiscovery, Search for content in a Core eDiscovery case and Keyword queries and search conditions for Content Search and eDiscovery
The following screenshot shows that I searched the mails with the sender "Lucas" from the specified mailbox:
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