1. Connect to exchange online using powershell. https://sutaantra.com/exchange/connecting-to-microsoft-365-online-exchange-powershell/
2. Once connected run the first command, this will enable automatic reply. Run the second command to check the status. To disable auto reply run the third command. For more information – https://docs.microsoft.com/en-us/powershell/module/exchange/set-mailboxautoreplyconfiguration?view=exchange-ps
- KB170063