How to mark email as unread in outlook folder?

If you want you can try using VbScript (as a last resort). This is for custom folder in Root,commented is for Inbox.

Unreading based on Subject (contains) by passing it via Start Process.

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Const olFolderDeletedItems = 3
Dim Items 
Dim objSubject
Dim Arg
Set Arg = WScript.Arguments
Set objOutlook = CreateObject("Outlook.Application")
Set objNamespace = objOutlook.GetNamespace("MAPI")
//Set objInbox = objNamespace.GetDefaultFolder(olFolderDeletedItems) --> Inbox (check)
Set objInbox = objNamespace.Session.GetDefaultFolder(olFolderDeletedItems).Parent._
     Folders("FolderName")
Set Items = objInbox.Items
Items.Sort "[Received]",True
MarkEmailUnRead(Items)

Sub MarkEmailUnRead(objItems)
	For Each objItem in objItems 
	  objSubject = Arg(0)
	  objsubject = objItem.subject 
	  if Instr(objsubject,objSubject) <> 0 then
	  objItem.UnRead = True
	  objItem.Save
	  Exit For
	  end if 
	 Next
End Sub