What happens when you send a letter to someone and that person has moved? Your letter will automatically get forwarded for the first 12 months after the service. But you won’t get the person’s new address.
Here’s how you can have the Post Office automatically send you the new address: Just place “Address Service Requested” on your envelope and you’ll get the new address – at the grand cost of fifty cents.
Think about adding the line to the return address on your envelopes. That is, unless you don’t want to get notified or pay the fifty cents.
You can see details and requirements here and here. The rules are also printed in the Postal Service’s Quick Service Guide, Publication 95, section 507d. There is also a “Quick Service Guide 507d, Additional Services, Ancillary Service Endorsements” here.