"I say, however, the rest is measurable in time,"
says Franco of Cologne in his seminal Ars cantus mensurabilis, written at some time in the 13th century. Franco not only measured rest but also the notes in between and developed a notation that let musicians tell the relative lengths of notes for ever more intricate compositions just by looking at the score. We still use a form of this "mensural" notation today.
Now, do you want to tell not only the relative but also and even absolute lengths of emails just by looking at the message score in Mac OS X Mail?
›› Peter's message is bigger than Mary's, and I can see it clearly in my Mac OS X Mail mailbox list.