Getting notice of non-delivery message? Here's why.
"Why am I getting notice of non-delivery of messages to people
I've never heard of, let alone ever sent email to?".
This is a side effect of common email viruses that have become so widespread. Many viruses "forge" the from address when it sends itself out automatically via email. That is, the virus "pretends" to be from someone that it is not really from. They use any address found in an Outlook address book on the infected computer as the forged from address.
The virus sends itself to other addresses it finds in that same address book. If some of those addresses are no longer valid, the mail will be returned to sender with a notice of non-delivery. This is commonly called a "bounced message".
The mail system sends this "bounced message" to the only address it has, the forged address. If the forged address is YOUR address, you will get the "bounced messages".