Sorry sandbox, your reply positioning confused me. I forget that the layout isn't so obvious.
It's not uncommon for a sent message to get stuck in the Outbox for no obvious reason, simply some kind of "curruption." The large file is an interesting point: once upon a time Entourage had a bug that could bring down an entire Exchange server if someone tried to send a message that was too large, but that was patched a long time ago. Nowadays a user should just receive an error from a mail server if the message was too large to be sent.
Re Entourage db size, note that Entourage X does indeed have a 4 GB db size limit, but 2004 and later do not:
http://www.entourage.mvps.org/database/size.html 
