ok, I had this working. I haven't looked at it for a while and it must have changed after doing some updates.
We are running Server 10.7.5
Open Server Admin
Select your server and drop the menu down
Select Mail
Select Settings
Go to Filter tab
at the bottom of the page there is an option to enable server side mail rules
Save
Now when you log in to Webmail and go to Settings and then Filters and add a filter there is the optino for reply:
Log into webmail
Click on Settings (top right corner)
Click on Filters Tab
Click on Add Filter (single page with green + icon)
Name the filter something like Vacation or auto respond or whatever
Click All Messages
Choose Reply with Message from the drop down menu
Fill out the form to say what you want (you can leave the last two fields empty)
Click Save or Add (both seem to create the Filter)
Far right there is a Filter Disabled check box for you to use to activate or disable auto response.