I have an email address book with lots of people I've worked with over the years. Occasionally, I'll remember someone in there and send them a "Hi, how's it going?" note just to keep in touch. If I don't get a reply, and it had been a particularly long time since I'd communicated with that person, I figured their email address had changed or something.
Lately it seems more of my notes have disappeared into the ether, and I think the explosion in spam is to blame. Either the recipients have mis-configured spam filters, or they just didn't recognize my name and tossed a note with a subject line like "Hello" or "Greetings". Sometimes I wonder if I've done that myself.