Salesforce sets the from address as each users own address by default. This means that responses from the clients go to the users own mailbox, not salesforce.
Instead, we would like the from address to be set as an email to case queue address by default so that responses are captured by the queue and processed directly into salesforce.
I can't find a way to do this though. I was able to make buttons that set the p26 parameter (the from address), but I can't find a way to override the reply/to all/send email buttons in the email message related list.