Why this is worth an hour
A postcard to a bad address costs exactly as much as a postcard to a good one. You pay for the print, you pay the postage, and you get nothing back.
On a 500-card drop at $1.19, a list that is ten percent bad wastes about $60. That is not catastrophic, but it is entirely avoidable, and the same ten percent quietly distorts every number you calculate afterward — your response rate is measured against 500 when only 450 cards could ever have arrived.
The checks below take an hour on a list of a few thousand and do not require any special software.
Duplicates
Duplicates are the most common problem and the most embarrassing one, because the recipient sees the result. Two identical cards arriving the same week reads as carelessness.
Exact-match duplicates are easy — sort by address and look for repeats. The ones that slip through are near-matches: the same household entered as "123 Main St" and "123 Main Street", or a married couple entered as two separate records at one address.
Decide deliberately whether you want to mail a household once or a person once. For most local businesses, once per household is right, and it is cheaper.
Missing and malformed fields
A record needs a street address, a city, a state, and a postal code to be mailable. A missing unit number on an apartment is usually fatal too — the card goes back or goes nowhere.
Sort your file by each column in turn and look at the top and bottom. Blanks sort to one end, and typos with stray characters sort to the other. This one trick surfaces most of the damage in a couple of minutes.
Watch for postal codes that lost a leading zero. Spreadsheets do this silently when a postal code column is formatted as a number, and it turns every New England address into a broken one.
Stale addresses
People move. Depending on your area, a meaningful share of any list more than a couple of years old points at an address where your customer no longer lives.
The US Postal Service maintains change-of-address data, and mailing services can run a list against it so cards follow people who moved rather than going to their old address. If you are mailing a list you have been keeping for years, this is the single highest-value pass you can run on it.
If you cannot run that check, the crude version still helps: sort by last visit date and look hard at anything older than about three years.
Check the file before it becomes paper
The last step is the one people skip. Before you commit to a drop, look at ten records at random and read them as though you were the mail carrier.
Does each one have everything needed to deliver it? Is the name spelled in a way you would be comfortable printing? Is anyone on this list someone you should not be mailing — a customer who asked not to be contacted, a competitor, your own office address?
Printed mail is unrecoverable. Once a card is in the mailstream there is no editing it, no pausing it, and no taking it back. Ten minutes of reading beats an apology.