Excel VBA - Macro Runs but has no effect - vba

Question

I'm new to Excel VBA and this is my first macro, so please forgive me if I've made a very obvious mistake. I have the following code to compare to worksheets and, if a match is found, to make a note on one of the sheets. It runs with no errors, but the changes are not being made. I can't see where I've gone wrong. Thanks in advance for the help.
Sub invalid()
Dim i As Integer
Dim j As Integer
Dim main As Worksheet
Dim invalid As Worksheet
i = 2
Set main = ThisWorkbook.Worksheets(1)
Set invalid = ThisWorkbook.Worksheets(2)
Do
j = 2
Do
If LCase$(invalid.Cells(i, 1).Value) = LCase$(main.Cells(j, 13).Value) Then
main.Cells(j, 14).Value = "Invalid Email"
End If
j = j + 1
Loop While main.Cells(j, 2) = Not Null
i = i + 1
Loop While invalid.Cells(i, 2) = Not Null
End Sub

Answer 1

Try this, it removes one of the loops:
Sub invalid()
Dim i As Long
Dim j As Long
Dim lRow As Long
Dim main As Worksheet
Dim invalid As Worksheet
Set main = ThisWorkbook.Worksheets(1)
Set invalid = ThisWorkbook.Worksheets(2)
lRow = main.Cells(main.Rows.Count, 13).End(xlUp).Row
For i = 2 To lRow
j = 0
On Error Resume Next
j = Application.WorksheetFunction.Match(main.Cells(i, 13), invalid.Range("A:A"), 0)
On Error GoTo 0
If j > 0 Then main.Cells(i, 14) = "Invalid Email"
Next i
End Sub