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 <a href=”http://itknowledgeexchange.techtarget.com/it-trenches/2000-users-new-mailboxes-one-weekend-done/”>this for our users</a> 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 <a href=”http://itknowledgeexchange.techtarget.com/it-trenches/did-you-see-this-need-some-exchange-advicesupport/”>blog</a>.