Because we were blacklisted some time ago, we've implemented a block on all outgoing vacation (a.k.a. Out Of Office) messages originating on the Lotus Domino servers. Apart from setting the checkbox on the agent in the mail template, the out of office agent adds a custom header to the outgoing message. This is necessary to catch out of office agents that have been set to run for a long time and also for the Domino Web Access (a.k.a. iNotes) users; our chaps haven't found the place to disable the agent in the template... Anyhows, all OOO messages are now caught on the Exim gateways, and they are silently dropped with a data ACL:

discard message = Discarding Out-Of-Office Message
      condition = ${if def:h_Xsomeheader: {1}{0}}

You won't be hearing from our Mr Clarks when they are away. ;-)