An autoresponder is a message that is automatically sent in reply to any email that lands in your inbox. For example, you might set an autoresponder before you go on holiday. When someone sends you an email, they will automatically receive your message back, which you can use to inform them when you will return to work, supply alternative contact details, or simply acknowledge receipt of their email.
For an Exchange 2019 mailbox, this is created through the Outlook Web App facility.
Link to OWA for Fasthosts customers: https://login.livemail.co.uk