FAQ I have my city, state and ZIP code all in one field, however when I presort my list it tells me “ZIP code is not defined? In order to run your list through the presort function of MAILERS+4 you must have a separate ZIP code field formatted in your list. When using a .DBF file, go to the “Modify” function under the “Database” menu. Insert a ZIP code field with a length of 10 and then format the field as “ZIP” in format fields.