As with other mail configuration, autoresponders are configured via the hosting control panel of the server that hosts your site. This will either be:
Once you've logged in, go to 'Create/admin email autoresponders'. Give the autoresponder a name that is not just your email account name (we suggest something meaningful like holiday or sick-leave, or whatever is appropriate) and fill in the message that you want to send.
At this stage, the responder is not active. When you want to activate it, go to 'Change mail alias configuration', select 'Edit' on the email address you want to apply the autoresponder to. The form will end with something saying AND '<None>'. Change that '<None>' to 'Forward to:', and put the email address of the autoresponder. For example: if you are on the yourdomain.com domain, and your autoresponder is called 'holiday', put 'firstname.lastname@example.org' in box next to 'Forward to:'.
If you have multiple aliases, e.g. <email@example.com> and <firstname.lastname@example.org>, then you'll also need to apply the autoresponder to them.
From then on, you'll continue to receive email in your inbox, but the autoresponder message will also be sent.
To deactivate the autoresponder, go to 'Change mail alias configuration' again, and change 'Forward to:' back to none. The autoresponder message will still be there ready for your the next time you need it.