"In regular direct mail, you can have mailers renting your list return undeliverable addresses to you."
"Addresses" are some unmovable physical locations, so "return undeliverable addresses" seems weird.
Not necessarily- what happens if, for example, an address on the list has since been demolished- if a number of houses have been replaced by a block of flats, how could they deliver? Empty houses sometimes have the doors and windows covered with metal grills to prevent squatters using the house. Houses burn down, roads are built, businesses close, etc.
I meant to say that it is probably more logical to "return undeliverable mail" back to the sender than "return undeliverable addresses" to the sender.
Not in this context- the person has rented a list of addresses that they are mailing to, but when mail is undeliverable, they merely pass on the address the the owner of the list, not the mail. The person can then update the list.