Maybe some values don't match. Try

[vba]

Const LOOKUP_TABLE AS string = _
"'T:\Operations+Technology\NOC\Abuse Emails\[Customer Data for Abuse Emails.xls]Report'!$E$4:$G$60000"
Dim lastrow as Long

With Sheets("Master List")

lastrow = .Range("A1").End(xlDown).Row

For i = 2 To lastrow

If Not IsError(Application.VLookup(.Cells(i, "B").Value, , LOOKUP_TABLE, 2, False)) Then

.Cells(i, "B").Value = Application.VLookup(.Cells(i, "A"), LOOKUP_TABLE, 2, False)
Else

.Cells(i, "B").Value = ""
End If
Next i
End With
[/vba]