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