The key here is to have your engineers create this rule in Outlook and use it instead of their Automatic reply rule when out of office.
1. The ticket owner creates an email rule which replies to any emails sent from the KBOX service desk to them, with an arbitrary value such as: "kref:ooo" and an ,except if, email contains the words, Owner Changed to.
2. You add, in Queue Customisation, Priority Values: called ""Engineer Out of Office"" and make this a different colour (Yellow)
3. The rule looks like this:
Note: Owner Full Name - <just leave this field blank>
So here's what happens:
If a submitter emails the ticket queue then the rule kicks in and changes the owner to <blank> and updates the Priority to ""Engineer Out Of Office"" thus dropping it back into the active and unassigned queue and highlights it for your other engineers that an OOO rule has kicked in and that someone has replied to an open ticket.
Clear as mud?