Voter caging involves challenging the registration status of voters and calling into question the legality of allowing them to vote. Usually it involves sending mail directly to registered voters and compiling a list from mail returned undelivered. Undeliverable mail is seen as proof that the person no longer resides at the address on their voter registration. The resultant list is then used by election officials to purge names from the voter registration rolls or to challenge voters' eligibility to vote on the grounds that the voters no longer reside at their registered addresses.