Replies and forwards to the Internet are off by default and for very good
reason.  To enable this, talk to your Exchange admin, and then walk away
when he/she starts laughing hysterically.

OOF to the internet is disabled by default in Exchange. Ask the admin if
he/she wants to open this up....many don't, as it will reply to all
spammers, and for various other reasons.

Because that is the default behavior for Exchange.  Your company isn't
interested in your particular absence being known to their customers.  An
inability to assign someone in your absence to handle any e-mails from
customers, even those to you, makes your company look inept.  Customers
don't care that you are out.  They want a response and they want it now.
Not when you get back from being sick, and not when you happen to plug in
while traveling.  Your company is also not very keen on replying to every
spam mail that targets your account to divulge that it is a valid and active
account to the spammer.  As far as the company is concerned, it is no one's
business that you are not at the company.  As far as the customer is
concerned, they really don't care about you and want SOMEONE at your company
to respond, and not some brain-dead auto-responder spewing back a message
"not here, go away".

If you want your auto-response to get issued to outsiders, you'll have to
ask your Exchange admin.  Good luck.  I doubt they'll bother.  It probably
goes against their company policy and tarnishes their image.

In the out of office assistant, add rule and tick "Reply with". Click on Template
and save your message.