If undelivered, please return to sender.

若无法投递，请退还寄信人。

youdao

It came back with "return to sender" stamped on it.

邮件被盖上退还邮寄人的印戳退回。

youdao

To ensure mail items be duly received by the department, and to avoid unnecessary delivery delay (e. g. return to sender), please pay sufficient postage before Posting mail items to this department.

为确保本署能收妥邮件，并免却不必要的派递延误(例如退回寄件人)，在投寄邮件前，请切记贴上足够邮资。

youdao

If you ask me, I think Elvis said it best about such messages: Return to sender, address unknown, no such number, no such zone.

FORBES: Elvis Presley Had It Right When It Came To Insider Trading

Although the sums involved are very small in terms of its annual turnover, the Royal Mail says the proceeds are used to recoup part of the cost of its National Return Letter Centre based in Belfast, which aims to return undeliverable items to the sender wherever possible.

BBC: How much does Royal Mail make from undelivered parcels?