Send email based on due dates

Try This variation to your code

Private Sub Workbook_Open()

   
    Dim i As Variant
    Dim OutApp, OutMail As Object
    Dim strto, strcc, strbcc, strsub, strbody As String
    Dim today As Date
    Set OutApp = CreateObject("Outlook.Application")
    OutApp.Session.Logon
    Set OutMail = OutApp.CreateItem(0)
    today = Date
    
    For i = 3 To Range("C65536").End(xlUp).Row
       ' If Cells(i, 43) <> "Y" Then
            If (Cells(i, 3) - 30) < today Then
                strto = Cells(i, 4).Value 'email address
                strsub = "PSR Contractor - Due date " & Cells(i, 3).Value 'email subject
                strbody = "Dear " & Cells(i, 2).Value & vbNewLine & "please update your project status" 'email body
                
                With OutMail
                    .To = strto
                    .Subject = strsub
                    .Body = strbody
                    .Send
                   ' .display
                                    
                End With
                On Error Resume Next
                Cells(i, 5) = "Mail Sent " & Now()
                'Cells(i, ) = "Y"
                
            End If
        'End If
    Next
    
    Set OutMail = Nothing
    Set OutApp = Nothing
    

End Sub