How do we limit the amount of bandwidth our e-mail server eats up?

There is nothing in Exchange that will do that.
You need to looking at something on your router to restrict the amount of bandwidth SMTP uses.

Simon.

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This isn't a function of the software, you will need to enable bandwidth control or qos (quality of service) through your router or firewall.  Typically you place an inbound and outbound rule on port 25 that limits bandwidth by percentage of total available bandwidth (defined somewhere in the router) or as a fixed number in kb.

You should be able to install the Windows QoS driver as a service to your Network card and use the TCP/IP properties QoS settings right on the server to control the bandwidth use.
Else, it depends on what kind of firewall/WAN router you have. If it is a Cisco router, then basic rate-limiting will work. There are many options for QoS traffic control on routers.

Irmoore,

How would I go about enabling and configuring QOS on my mail server's NIC?  Thanks.

Irmoore,

I have a PIX 506e and Cisco says it doesn't have QoS capabilities.  Are rating limiting capabilities and QoS used synonymously in your post, or can I look for rating limiting functions even if the firewall isn't QoS compliant?  Thanks.