Modity on current code to prompt due date & auto email

Dear All,

I have below code from one of the pro in this forum.
I would like to modify this code into others worksheet.
Could some one explain to me

1.

For i = 3 To Sheets("EXPORT").Range("e65536").End(xlUp).row

What is the "i" stand for?
Range("e65536") means what?

2.

If Cells(i, 28).Value <= Date + 0 And Cells(i, 29) = "" Then

Date + 0 will prompt you the due date. What about if i wan macro to prompt me 3 days after due date. How should I amend?

Option Explicit
'Sub Auto_Open()

Sub CheckForExpiryDates()
Dim ce As Range, i As Long
Dim OutApp As Object
Dim OutMail As Object
Dim strto As String, strcc As String, strbcc As String
Dim strsub As String, strbody As String
For i = 3 To Sheets("EXPORT").Range("e65536").End(xlUp).row
If Cells(i, 28).Value <= Date + 0 And Cells(i, 29) = "" Then '28 = Due Date , 29 = Payment
Set OutApp = CreateObject("Outlook.Application")
OutApp.Session.Logon
Set OutMail = OutApp.CreateItem(0)

With Sheets("EXPORT")
strto = .Cells(i, 44).Value '44 = email address
strcc = "" '.Cells(i, 44).Value
strbcc = ""
strsub = "AutoMail: NON PAYMENT"
strbody = "Hi" & " " & .Cells(i, 13).Value & vbNewLine & vbNewLine & _
"The Bill for Invoice no." & " " & .Cells(i, 3).Value & " " & _
"; Customer :" & " " & .Cells(i, 1).Value & " " & _
"is due on " & .Cells(i, 28).Value & _
" " & vbNewLine & vbNewLine & _
"Please request your customer to expedite payment!!!" & _
vbCrLf & vbCrLf & _
"If shipment have been arrange, please inform the PIC to update shipment date." & _
vbCrLf & vbCrLf & _
"Thank you."
'.Cells(i, 16) = Now()
End With

With OutMail
.To = strto
.CC = strcc
.BCC = strbcc
.Subject = strsub
.Body = strbody
'.Send
.display

End With

Set OutMail = Nothing
Set OutApp = Nothing
End If
Next i
End Sub

TIA!!!

1. puts to row 3 to last row with data in column E
2. not sure what Date + 0 does to code

to no break the mailing part of the code probably put an IF loop before existing if loop
for msgbox "3 days till due"

ok date is for todays date

so have

If Cells(i, 28).Value +3 <= Date And Cells(i, 29) = "" Then '28 = Due Date , 29 = Payment
msgbox "3 days til due"
end if

Sub CheckForExpiryDates_N_Email()
Dim ce As Range, i As Long
    Dim OutApp As Object
    Dim OutMail As Object
    Dim strto As String, strcc As String, strbcc As String
    Dim strsub As String, strbody As String
    For i = 5 To Sheets("EXPORT").Range("e65536").End(xlUp).row '5 = fifth row
        
        If Cells(i, 28).Value + 3 <= Date And Cells(i, 29) = "" Then
            MsgBox "3 days till due"
        End If
        
        If Cells(i, 28).Value <= Date + 0 And Cells(i, 29) = "" Then      '28 = Due Date , 29 = Payment

have the rest of the code as is

nice touch with the

MsgBox "No Over Due Found! " & vbCrLf & "Payment Received In Time *\(^_^)/* "

i looked at your sample
reason nothing works is your column E is empty and that is what the whole loop functions,....unsure why E was chosen, whoever coded it must have their reasons

i assume your code was working fine before?

amended so it works
if within 3 days it will msgbox you
if overdue it create will email