If your list includes missing zip codes, you mailing list can also use a reverse zip code custom function to return the zip code for a given mailing list city and state. Another common problem is that leading zeros can be dropped when zip codes are entered in Excel. This can be prevented by changing cell formatting to text before the data is imported.
But if you've already created your list, just use a custom function to automatically return valid 5 digit zip codes from the database, with all mailing list leading zeroes intact. Custom functions are ideal for processing long lists of addresses. There is no need to enter a new formula manually for each item in your list. Just enter a single formula that includes the custom function and a reference to the first zip code.
Then copy this formula down your entire list - results for thousands of addresses can be returned in seconds. This is a huge advantage over mailing list transferring data one-by-one from the Post Office web site or other zip code software. Custom functions can provide many different kinds of information. They can be used to calculate distances between zip codes or zip codes within a given radius.