On occasion, you may have a package that is unable to complete its initial delivery. This can happen as a result of invalidated addresses, over-sized or uniquely shaped packages that do not fit in a recipient's mailbox. Other factors that can lead to redelivery are additional options that require someone to be present at time of delivery, such as a signature confirmation.
In these circumstance, the USPS mail carrier will leave PS Form 3849, a specific document intended to notify the recipient that their package has failed to be delivered. The form describes the reason for non-delivery and invites the customer to take action on retrieving it. The amount of time the package will be held and instructions on how to acquire it is also indicated on the form is. If the package is not claimed during the indicated held time, it will be returned to the sender.
While many of these circumstances for redelivery can't be predicted, making sure to validate all addresses and communicating any special terms of delivery with your customer will help avoid instances of redelivery.