Hi @English, Tabbie ,
Welcome to Q&A forum!
According to my research and testing, when the mail is dragged into the library, the default file creation time of SharePoint is the time of dragging. To get the send on timestamp, you need to get the content of the email, which is currently no ootb way to make it in SharePoint. Maybe it's possible with Power Automate. but Power Automate is currently not supported in the Q&A forum. So we suggestion you go to Power Automate Community create a new discussion to get more professional help.
Hope your issue gets resolved soon. :)
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