Message lost in cyberspace?

This is mostly a PSA post repeating something that this blog and others have mentioned:
if you're expecting some piece of work from people (i.e., you're organizing a panel, editing a book collection, sending out manuscripts for review, and so on), please send a quick message to acknowledge its receipt. You don't have to write anything fancy--"thanks" will do--but write something.

So far, in the past few weeks, I've had nothing but complete silence after sending the following: repeated messages to a possible panelist for a panel I'm organizing, a manuscript review, a message to a student who hasn't shown up to class in a month advising her to drop, and a proposal for a conference panel. I usually send a copy to myself at a different address the same time just to be sure that the messages all got sent, and they did.

After two messages, of course, sometimes people will respond by saying, "Yes, I got your message and the earlier one, too." Then why on earth didn't you respond the first time?

Why don't people respond? My current theories run from the sane (they didn't get the messages) to the considerably less than sane (they gave up e-mail for Lent? they're conducting an elaborate psychological experiment in which I'm an unwilling, non-IRB-approved participant?).

Maybe they enjoy all the attention I lavish on them with repeated messages. Maybe they're suckers for those obnoxious little red exclamation points that flag "important" messages and are waiting for me to use them. Or maybe they think I'm not really serious because I don't use the delivery confirmation feature.

Whatever the reason, unless the e-mail's from the long-lost son of a Nigerian dictator who's just waiting to wire you millions once you give him access to your bank account, it's polite to answer the e-mail.