Yes, you can filter list2 based on list1 using the VLOOKUP function in Excel. Here's how you can do it:

1. Open both lists in Excel. Make sure that both lists are sorted by customer numbers.
2. In a new column next to list2, enter the following formula in the first cell: =VLOOKUP(A2,list1,1,FALSE)
   Note: Replace "A2" with the cell reference of the first customer number in list2.
3. Drag the formula down to the last cell in the column. This will populate the column with either the customer number (if the customer is active) or #N/A (if the customer is inactive).
4. Select the entire table (including the new column with the VLOOKUP formula).
5. Click on the "Data" tab in the ribbon, and then click on "Filter". This will add filter arrows to the top of each column.
6. Click on the filter arrow in the new column with the VLOOKUP formula. Uncheck the box next to "#N/A" to hide all inactive customers.
7. Now you should see only the active customers in list2. You can copy and paste this filtered list to a new sheet or document to use for your letter.