Sparkpost does not seem to have ipv6 connectivity on their outgoing MTAs. This prevents ipv6-only servers from receiving the registration mail (or any mail from this site). I tested this using my own mail server, disabling its ipv4 connectivity and registering this account.
This is not so much of a bug report as a feature or future-proofing request, as I doubt a significant amount of people run mail servers without ipv4. Also, unless Sparkpost shows interest in addressing this, I fear there may not be a trivial solution.