You're going on holiday / vacation, or maybe a longer break like a Sabbatical, and you don't want a month's worth of emails waiting for you when you get back - so you want to turn off your email and send an auto-response to anyone who emails you.
The solution is not at all obvious.
If you use cpanel to suspect incoming emails, users get a nasty 550 message and no auto response.
If you use a filter to discard the email or redirect to a folder, the autoresponder won't work.
Here's what you need to do:
1. Set up an autoresponder for a made-up email on your domain. I just stuck a 1 on my email - so if my real email was email@example.com I made an autoresponder for firstname.lastname@example.org. Make sure the "from" field has your main email in it (email@example.com)
2. Set up a filter on your normal email account. This should have two actions:
(i) for all "has not been previously delivered" emails, redirect to the new autoresponder email (firstname.lastname@example.org),
(ii) discard the email
That'll do it. Why you can't just have the autoresponder on your email and then discard the email I don't know...