After years with Thunderbird, I decided to give Mail a serious look. To avoid boring everyone, I'll just say that I'm quite happy. One thing I noticed, though, is that Mail doesn't seem to be aware of "read receipts" in any way. This comes as a big surprise to me, since my introduction to the idea of read receipts came from the very same Mail.app (13 years ago when OS X was still called NeXTstep).
Anyway, I wasn't interested in emulating full read receipt support as discussed in a previous hint, but I was curious to see which of my control-freak contacts were requesting them. The following will allow you to more easily see if a read receipt was requested.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060303082553887