Hi @Yiru Chen
According to SharePoint online does not have Incoming E-mail feature, I suggest you create a shared mailbox for document library, then create a Microsoft flow to import new mail from the shared mailbox into document library.
1.Create a shared mailbox for document library. Go to Microsoft 365 admin center -> Exchange -> Mailboxes -> Add a shared mailbox as following pictures show.
2.Change the password of the shared mailbox. Go to Microsoft 365 Admin Center -> Users -> Active users -> Click your shared mailbox -> Reset password.
3.Create an automated Microsoft Flow as following pictures show.
(1)Add a new step -> Choose ‘’Export email’’ -> Choose ‘’Message Id’’ (remember change connection to your shared mailbox).
(2)Add a new step -> Create file -> Site address (your SharePoint site address) -> Folder address (your document library address) -> File name: Subject .eml -> File content: body -> Save (remember change connection to your SharePoint account).
4.When you send an email to the shared mailbox, there will be an .eml file created in the document library.
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