Nearly every day my physical mailbox is filled with magazines and advertisements. Not only is this horrible for the environment, but it prevents important mail from being delivered to me when I am on vacation or for any other reason when I miss checking my mail on a daily basis.
Today, the postal service will return first class mail that is marked 'return to sender'. They will not do the same for presorted mail (the primary type of mail used by Junk Mailers). This leaves pre-sorted mail as a one-way spam system for anyone that doesn't want to be accountable and have their mail rejected by recipients.
Even worse, few companies honor the DMA opt-out program.-- Even if they do, they are using exact name/address matches for opt-opt, which leaves 100s of name/address variations open to spam.