VB Code To Email Separate Sheets

below is a VB code to email separate sheets based on sheet anme and email address on a sheet called " mail".

Code seems to fail at the last line " Next a" any ideas why this is not working

Sub Mail_sheets()
Dim MyArr As Variant
Dim last As Long
Dim shname As Long
Dim a As Integer
Dim Arr() As String
Dim N As Integer
Dim strdate As String
For a = 1 To 253 Step 3
If ThisWorkbook.Sheets("mail").Cells(1, a).Value = "" Then
Exit Sub