Out of office , iPhone 13 max pro, iOS 16.5?

Tap on the Home icon at the top of the Outlook app, tap on your account in the list, tap on Automatic Replies, and turn it on. Then set up a reply in the text box provided. You can set a time frame for that reply to be active.

More info at Microsoft's website:

https://support.microsoft.com/en-au/office/how-do-i-enable-out-of-the-office-messages-for-each-of-my-accounts-4b9edc3b-9968-4d66-8c4f-77fd5f91f553#:~:text=In%20Outlook%20for%20iOS%20and,then%20toggle%20the%20switch%20on.

There is no option for this on the iOS mail app, you need to use the Outlook App.