Is this on an Exchange Server?

During a Run in the VBE, press ESC key to abort macro after a bit. Look at the Immediate Window. The To names are shown but not the email address. If you use SenderEmailAddress on Exchange, that is not helpful.

IF you can live with the .To names, we can go from there.

This is all you need in a Module:
Sub SendReminder()  
  Dim v, item, i, ws As Worksheet, r As Range, c As Range, cc As Range
  'Tools > References > Microsoft Outlook xx.0 Object Library > OK
  Dim OL As Outlook.Application, oMI As Outlook.MailItem, oFSentItems As Outlook.Folder
  
  Set ws = Worksheets("SendReminder")
  Set r = ws.Range("I2", ws.Cells(ws.Rows.Count, "I").End(xlUp))
  
  Set OL = CreateObject("Outlook.Application")
  Set oFSentItems = Outlook.GetNamespace("MAPI").GetDefaultFolder(olFolderSentMail)
  If oFSentItems.Items.Count = 0 Then Exit Sub
  
  For Each c In r
    Set oMI = Nothing
    Set cc = ws.Cells(c.Row, "P")
    If cc > Date Or cc = "" Then GoTo NextC
    For Each item In oFSentItems.Items
      If TypeName(item) <> "MailItem" Then GoTo NextItem
  Debug.Print item.To, item.Sender, item.ReceivedTime, item.Reply.SenderEmailAddress
        If oMI Is Nothing Then Set oMI = item
        If item.ReceivedTime > oMI.ReceivedTime Then Set oMI = item
NextItem:
    Next item
NextC:
    If Not oMI Is Nothing Then
      With oMI.Reply
        .Subject = ws.Cells(c.Row, "Q")
        .Body = "Dear " & ws.Cells(c.Row, "R") & vbCrLf & vbCrLf & _
          ws.Cells(c.Row, "S") & vbCrLf & vbCrLf & ws.Cells(c.Row, "T") & .Body
          If ws.Cells(c.Row, "U") <> "" Then .Attachments.Add ws.Cells(c.Row, "U")
        .Display
      End With
    End If
  Next c
End Sub