GLOSSARY
Nixie
By Justin Winter · Updated August 28, 2026
Postal-industry slang for a piece of mail that cannot be delivered as addressed, most often because of a bad or incomplete address.
A "nixie" is undeliverable mail pulled aside by USPS for special handling — the term predates automation and survives as informal industry shorthand across carriers and mail-services vendors, not just USPS.
It describes the same underlying problem that "undeliverable as addressed" names formally: the address on the piece does not resolve to a real, reachable delivery point.
See also
- Undeliverable as addressed — USPS’s classification for a piece that cannot be delivered to the address written on it — the umbrella term behind most returned mail.
- Mail Recovery Center — The USPS "dead mail" office where undeliverable mail goes when it also cannot be returned to the sender.
- Address standardization — Rewriting an address into the exact form USPS uses — canonical abbreviations, directionals, unit designators, and ZIP+4.
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