I did some more testing. It turns out that it only looked like it was a size problem.
I believe that you are on the right track with the keyword issue.
As I said earlier, my message is nothing more complicated than a list of 100+ links.
One of them was similar to this:
<a href="http://www.wgt.com/clubs/the_peoples_club/forum/t/479035.aspx">U.S. Open</a>
When I removed the link and just left "U.S. Open" in there as plain text, I still got the error. That's why I originally thought it was a size issue.
If I replaced the "U.S. Open" text with "12345678901234567890", it worked fine. Hmm. That's even longer. Why didn't I get the error?
I reverted back to the link and replaced "U.S. Open" with "U.S. Open". It worked.
Strangely though, if I create a new message containing only the original link (with the space), it also works.
Javascript is one thing but I have no reasonable explanation why "U.S. Open" would cause any sort of a problem.
I guess we just have to guess at what the offending code is until there is some resolution to the problem.