Items are returned to sender if:
The address does not exist.
The item does not bear a sufficient address.
The addressee has moved without providing a forwarding address.
The item is refused by the addressee.
The item contains insufficient postage.
Your item will require new postage before you resend it.
What you can do:
If the item was returned to sender in error, contact the couriers to report the issue.
Learn how to properly address your letters and parcels.