Well, I'm sorry to say that you're moving into the realm of the untried
here!
Have you downloaded those 270,000 mails, or are they sitting on your server?
If you have downloaded them, the best advise I can offer is to reboot the
mac, open just entourage and then try again.
If they are still on the server, try to access the inbox through web mail or
whatever other method your ISP provides - you may be able to delete a mass
of stuff that way.
If you do get it going again, you need to start learning to use rules that
can automatically deal with this stuff:
http://www.entourage.mvps.org/rules/index.html
I will ask elsewhere is there is any other suggestions, but I wouldn't hold
out much hope,