Google doesn't currently send out email over ipv6, and your only MX record points to a name that only has an AAAA record, not an A record.
Last time I checked, none of the big3 did (google, yahoo, hotmail/msn).
Your ipv6 is from HE, so it would be doubtful your isp would be blocking smtp on tunnelled traffic.
I can hit port 25 on your mail server from another HE IP:
220 WIN-327SUH9SP8Q.miklasz.co.uk Microsoft ESMTP MAIL Service ready at Fri, 15 Jun 2012 20:36:59 +0100
I've never used any, but there are several autoresponders out there that are dual stacked or ipv6 only, here's one I found from a quick google query:http://www.v6address.com/node/14
It says it will respond to anything you send it with the headers of the email you sent, so should be good for troubleshooting/verifying your setup.