It is very important these days to keep a small attack surface, both personally and professionally. When you get a “User unknown” message back, delete that person from your Email distribution lists. Too many companies keep names in their lists forever. That is a really bad idea.
When a domain name expires, it is very common nowadays to find that some miscreant has attached a mail server to it. It is either blindly accepting all mail, or it might even be rejecting them. But, IN ALL CASES, the sender’s Email address, YOURS, is being harvested.
There are tons of misguided individuals and entities that would love to get their hands on your email address. There is so much more to an Email address than unwanted Spam. These days, an Email address is half of the key to most online sites, like Dropbox, Google, Facebook, Twitter, Tumblr, Macy’s, Amazon, Walmart, iTunes, Microsoft, BestBuy, Box, Hulu, Spotify, Netflix, etc.
Minimize your online footprint. Don’t be lazy. You are going to get burned.