I recently acquired a DLO postcard. This particular post card was posted from Ahmednagar City on 12 Feb 1897, destination Bombay. I am not sure how DLO works because I see another Ahmednagar postmark that overlaps the DLO slip pasted on the postcard. Not sure if they actually sent the letter back to the destination city even as the return address was not known.
“A dead letter office (DLO) is a facility within a postal system where undeliverable mail is processed. Mail is considered to be undeliverable when the address is invalid so it cannot be delivered to addressee, and there is no return address so it cannot be returned to the sender.
At a DLO, mail is usually opened to try to find an address to forward to. If an address is found, the envelope is usually sealed using tape or postal seals, or enclosed in plastic bags and delivered. If the letter or parcel is still undeliverable, valuable items are then auctioned off while the correspondence is usually destroyed. Despite this practice, in the past some undeliverable envelopes were acquired by philatelists.
Dead letter offices go by different names in different countries. Other names include returned letter office or undeliverable mail office.”