in addition to the smtp mail send by he.net I would suggest to add a imap request to retrief this email.
ipv6 secured by tls
possible implementation:
imap-server -> dovecot
imap-client -> kmail / thunderbird
no idea how to handle useraccount/password handling ...
I don't think your suggestion is really testing peoples ability to setup IPv6. Rather it is more a test of their ability to setup IMAP and TLS, which is not the purpose of the certification.
This is very different from testing the ability to run SMTP over IPv6. The point is that if you are using email, you are using SMTP. That's just how email works. You just have to be able to do that over IPv6 rather than only IPv4. But you can very well be using email without having IMAP being used anywhere in your setup, and even if you did use IMAP it wouldn't have to be going over the public Internet.
So now you'd require people to first setup IMAP, and really that would be the significant part of the task. And TLS would probably mean people would have to go and buy a certificate or do research on how to get one for free. But the process of acquiring a certificate doesn't really have anything to do with IPv6 either.
That was my thought too