- The person addressed moved and did not file a change of address with the post office
- An apartment number or suite number was not included as part of the address.
- An internal mailstop number wasn't included - some large companies require this before their internal mailroom will deliver it.
Mailing at 1st-Class rates can result in mail being returned and not being deliverable as addressed. There can be a lot of reasons why mail could be returned as not deliverable. Some of the biggest reasons could include: