Be sure to use Outlook 2007 in cached user mode. This will definitely reduce network traffic. One suggestion is to put attachment size limits on the messages. We did this for our users and this does seem to also help manage network traffic. Note that a lot of organizations restrict attachment sizes to around 5-10MB. Teach your users to zip their attachments also.
ADSL could be an issue for sending e-mails from the remote sites. The upload speed is usually significantly lower than download speed. Your users will likely experience poor sending performance. I recommend making the links symmetric or going to a T1 or something similar.
Some good Exchange resources can be found in my blog.