Exchange Bandwidth Issue

Okay, here's the situation - Win2003SP1 AD domain with Exch03SP2. We have a DMZ with ISA04 as the "back" firewall with Exchange sitting behind it. We also have a web server sitting in the DMZ tied to a DB server behind the ISA server. Last night a user sent 1100 messages that were 1MB in size and the entire network choked. We have folks working an event who needed access back into the webserver/database and were basically locked out until I turned off the SMTP service on the Exchange server (I'd used Network Monitor to see that the traffic was originating from Exchange). I turned the service back on in the middle of the night to attempt to clear the queue while no one would be needing network access (which seemed to work).

Here's the problem: it's going to continue happening. These email blasts (not spam, this is to members who've asked for it!) happen a couple times a week now. I can't have these blasts choking the network.

Looking at Task Manager, Networking on both the ISA and Exchange boxes, they claimed about .18% of the bandwidth of the NIC was being utilized. I know that isn't the most accurate tool in the world, but still...

What can I do to make better use of the bandwidth I've got? And are 1100 1MB messages unreasonable? Our infrastructure is 100 megabit between the Exchange server and the ISA server. Is there a "buffer" modification I can make to ISA or Exchange?

This probably isn't a hard question but it is important to me. Thanks much.