Why should anyone use this service over something like Gmail, which hopefully around W6L have IPv6 enabled SMTP once all the white-listing is gone?
I am hoping as well, but I haven't seen any signs suggesting Google will have IPv6 support in Gmail ready a month from now.
It is not like the whitelisting was applicable to SMTP in the first place. If you claim to support SMTP over IPv6 people won't accept mail delivery failing just because the sending mail server for some reason wasn't allowed to know about the existence of an AAAA record. So it would be all or nothing. The only meaningful way to do a staggered rollout would be one domain at a time. And I guess the majority of Gmail users are still using the gmail.com domain.
And whitelisting wouldn't really solve as many problems for SMTP as it did for HTTP (the number of problems whitelisting solved for HTTP is still a subject of discussion). For SMTP there are better ways to work around clients with really broken IPv6 access. Create multiple MX records at different priority. Make the top and bottom priority IPv4 only, and those in between dual stack. It is a reasonable assumption that the broken senders will start at either the top or bottom priority, and with that being IPv4 only, the IPv6 brokenness shouldn't be a problem. And SMTP has always had higher requirements for retries than HTTP. An IPv6 only sender will still be able to deliver email as long as it tries all MXes in priority order, the way it is expected to behave.
The only reason to get our hopes up for Gmail supporting IPv6 after the launch day is that once dual stack is launched for majority of websites, remaining IPv6 brokenness will decrease further and completely eliminate that as an argument for not supporting SMTP over IPv6. Though frankly, I don't recall IPv6 brokenness ever being used as an argument for not supporting SMTP over IPv6.