Is the recipient on your Exchange server?
You can add your mailbox to the recipient full access list, then you will could open this mailbox and check message header.
You can also use Search-mailbox command to export target email from this recipient to your mailbox , then you will could check this email from your mailbox directly:
Search-Mailbox -Identity recipient -SearchQuery "Subject:Test1234" -TargetFolder check -TargetMailbox administrator
If the recipient isn't on your Exchange server, you will cannot check the message header with above ways.
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