> "[IPv6:2001:etc:etc::192.etc.etc]"

I'm trusting this is a throwaway example and that you used a real IPv6 address literal in this test, without the "IPv6" and with only colons and no dots (unless you mean to use v4 mapped address with dots)? Because this IPv6 literal is so malformed that I'm hardly expecting it to do something sane and changing that to "@2001" is nasal-demons quality undefined behavior. I tried with this exact literal and it let me send it but then there was a tiny red pop-up at the top of the gmail interface that said "could not be delivered, check your network connection" (which is odd; the same kind of pop-up that appears in gray when you legitimately are not connected to the internet) and it ended up in my drafts with the To: field empty.

I just tried to send a message to a "test@[" my current IPv6 address "]", and gmail told me

    Error
    The address "test@[«redacted»]" in the "To" field was not recognized.
    Please make sure that all addresses are properly formed.

This address doesn't have an MDA listening on it, but it didn't accept it enough to give me a non-delivery notification, it didn't even let me send it. gmail did accept an IPv4 address literal in brackets, although it hasn't given me back a non-delivery notification. What it stuffed into my Sent folder for this message has the square brackets stripped and the IPv4 address appears right after the @.