When one of your agents is out-of-office and you wish to notify the customer about the same when they reply, you can set up an automation rule under Admin > Workflows > Automations > Ticket updates > New rule:
When an action performed by...
Involves any of these events:
Reply is sent
On tickets with these properties:
In tickets > If Agent > Is any of > Saul
Send email to requester > customize your email