We actually don't send the full content in emails, just the HTML required to display the listings. Most of our "new listing" emails are around 50KB. Compared to the regular "hum" of users visiting sites, emails are hardly a blip on the radar. We currently run anywhere from 2Mb/s to a peak of 30Mb/s in the afternoons (2pm) and late evenings (8pm).
I know a lot of people hate the ConnectMLS method -- we have a number of Realtor clients just because of the way their system works.
-Matt