What determines if an address is deliverable or undeliverable?

Why is my mailer marketing being marked as undeliverable?

USPS validates all addresses in Invelo and determines if they are deliverable or undeliverable. Addresses are marked deliverable or undeliverable when imported or when a skip trace is done.

To see the deliverability status of the contact's mailing address, you click on any record, go to the 'Contacts' tab, click the drop down arrow below their name, and look for the status below their mailing addresses.