|While there isn't a filter or script I know of, there is a fairly simple way to do this...|
First, launch the compose window and compose the reply you want to send to everyone. Save this message as a template. Next open the mailbox that has the messages that you want to reply to. Click in the mailbox index pane and press Ctrl+A to select all the messages. From the "Message" menu select "Reply with Template" and choose the template you just created.
- This technique will result in multiple messages to the same sender if that person has sent you multiple messages and they are all in this folder.
- You can use the "Show only" bar in conjuction with this technique to filter the messages that you reply to.