Setting up an out of office message for email

it's a mail provider feature really, as if your computer is turned off something else has to send that auto reply and your computer being off ..

try reading

how to set an out of office message - Apple Community

more

mac Setting up an out of office message for email - Google-

April-May-June wrote:

how do I set up an 'out of office' message on email?

FWIW, I would not set up an auto 'out of office' email reply for two primary reasons:

1. If the received email is from a spammer or phisher your auto reply will confirm to the sender that they reached a valid email address. The barrage of spam and phishing to your confirmed email address will increase exponentially.
2. Your auto reply may result in an infinite loop with the sender if the sender also set up an auto reply.