My ex-husband and I haven't lived together for over 4 years now and don't even share the same last name anymore. However, recently his mail has started getting delivered to my address. He is still getting mail at his address as well but this is annoying since I really don't want to be associated with him any more than I have to.

Any idea why this is happening all of a sudden, and how I can put a stop to it? So far I've just been writing "no such addressee" on the envelopes and putting them back in the mailbox, but it just keeps on happening.

If it is junk mail, it is because your address is listed on his credit report ( I had the same problem with my ex).  If it's "real" mail it may be because someone is trying to locate him and found your address on his credit report or he has given out your address in order to avoid someone.  If you continue to receive mail from the same people or companies, I would write them a letter and tell them that no one by that name lives at that address and stop sending mail or you will contact the postmaster general.  That should help. 

Actually, just write unknown on the envelope and put it back in your mailbox. Your carrier should pick it up and take care of it. It would also not hurt to drop a note to the carrier that he does not live there and you do not want to receive his mail or be responsible for it.

If you write "Return to Sender, Address Unknown, No Such Number, No Such Name" as I did, be sure to take a black marker and completely cover the bar code markings on the envelope. The postal clerk told me that if the bar code remains, the letter will be redelivered because of automated processing.

The postal clerk has a stamp which can be used for "Return to Sender" if you want to interact with her or him.

The big deal is to obliterate that bar coding at the bottom of the envelope and to write: "Not at this Address."  Then, toss the item in the collection box. This applies to first class mail. I don't know about magazines and other periodicals.

I know that we get mail addressed to someone that hasn't lived here in 15+ years.  I know because I've been here for nearly 11!  and they are all related to flying so this guy must have a pilot's license.  It frustrates me to no end when we have a sub. carrier because they leave this crap and I have to put return to sender and all that on the envelope.