It's BES. I think it has something to do with the fact that this one sender is an AOLer. Though I don't know exactly what causes the problem, I suspect it's an AOL/BES glitch, probably with some AOL proprietary nonsense behind it.
I took the very same email from my PC and forwarded to myself (therefore, no AOL involved) and it had a number of "more available(s)." Each time I hit those, the BB "auto more'd" the rest of the message until it was complete.
Not sure what this glitch is but anyone sending mail from AOL deserves not to get it all read.