Yeah, GMAIL doesn't send out email over IPv6
That's strange. I thought sending email over IPv6 would be easier for Gmail to support than receiving, and the only reason it wasn't supported a long time ago was in order not to confuse people by supporting IPv6 in one direction and no the other. But Gmail has supported receiving email over IPv6 since the 6th of June.
I'm not sure what's the reason for the lack of error messages. Maybe it just keeps retrying hoping that an IPv4 MX will show up. I tried a few experiments.
IPv6 only mx: No response so far.
mx pointing to domain with no A and no AAAA record: No response so far.
mx pointing to nonexisting domain name: No response so far.
Recipient domain itself nonexisting: Mail returned instantly. (When clicking "send", the bounce arrived on my phone before the webserver replied to the browser).