ldapvacation
A good number of _vacation_ solutions exist for the Exim MTA, most of which rely on users being able to log on to the machine on which Exim is running. A nicely documented solution titled Vacation Processing for Exim uses Exim's _autoreply_ driver and a vacation.msg file residing on the file system.
Since our Exim servers only have virtual users as in a _mail toaster_ (i.e. all information regarding users is contained in an LDAP directory and the user's are not able to log on to the system) I implemented a solution based on Marc Haber's ldapvacation, a tool which gets a message piped to it via _stdin_ and which produces a vacation or out-of-office message if the user is on vacation.
A small PHP interface to update the vacation interval and message on the central server as well as a Thunderbird extension to display the current status of the vacation setting rounds it all off.