When I receive misdirected E-mail, it almost always results from someone selecting the wrong David or wrong Ross from their address book. That is, both the intended recipient and I are both known to the sender. The sender's address book is organized by names, not by E-mail addresses.
I used to get phone calls in the middle of the night for a David Ross who was an attorney, either in private practice or in the District Attorney's office. The caller would be drunk and picked out the wrong David Ross from the phone book. Again, this was a problem with my name, not with my phone number.
There are apparently many, many David Rosses. I have met two others face-to-face, both times in doctors' offices. I have exchanged E-mail with several others. I even created a Web page about this situation at http://www.rossde.com/Ross.html.
How do I handle misdirected E-mail? On the first occasion, I reply quoting the original message. I tell the sender they have the wrong David Ross. If there is one of those caveats about condfendiality and deleting misdirected messages, I also inform the sender that such warnings are unenforceable, that the sender must bear full responsibility for ensuring correct addressing of such messages.
On subsequent instances from the same sender, I use a small application that returns the message in a format that indicates the stated E-mail address is invalid. That is, the message will appear as if bounced. If that does not work, I finally threaten to make any subsequent messages public by posting them on a newsgroup.