A virtual $10 says their site (I haven't looked) advertises the limit with how many millions of e-mails, and how many thousands of MP3's that is.
I'd also be willing to speculate that right after it says how many MP3s that is, there is a * and at the bottom of the page in small print it says " *This is assuming the mp3 song is 2 minutes long with a 64 bit rate" (I haven't looked either)
