noun
lost mail; undeliverable mail
Mail that cannot be delivered to the intended recipient, often due to an incorrect or incomplete address.
住所が間違っていたため、迷子郵便になってしまった。
Because the address was wrong, it became undeliverable mail.
Compound of 迷子 (まいご, 'lost child') and 郵便 (ゆうびん, 'mail'), referring to mail that has 'lost its way'.