Setting up an out of office auto reply in Outlook

For a Gmail account, use the vacation responder in Gmail settings on the web - that one runs on Google's side, so it works with the computer switched off. There's also a rules trick in classic Outlook to fake an auto reply, but it only works while Outlook is open and running. Not much use if you're on holiday with the laptop shut.

depends which Outlook. New Outlook or Outlook on the web: Settings (the gear) > Accounts > Automatic replies. Switch it on, tick the box to only send during a time period, set your dates and write the message. There's a separate box for people outside your organisation if you want a shorter one for them. Classic Outlook: File > Automatic Replies. If you can't find that option at all, your account is probably plain IMAP/POP (Gmail, a host's email etc). Automatic replies there have to be set up on the provider's own webmail, Outlook can't do it server-side.