Undeliverable is a term used to describe something that cannot be delivered. Similar words to undeliverable include non-deliverable, unsendable, unrecoverable, unshippable, infeasible, unviable, impractical, and impossible. These synonyms are often used in the context of mail and packages that cannot be delivered due to a wrong address or other delivery issues. For businesses, the term undeliverable can also refer to marketing communication that cannot be delivered to customers because of incorrect contact details. As such, it's important to know the various synonyms of undeliverable to understand the reasons for undeliverable delivery and communication, which can help businesses improve their delivery methods.