It looks like even gmail.com is now advertising SMTP over IPv6:
lucab@galatea:~$ dig mx gmail.com
;; QUESTION SECTION:
;gmail.com. IN MX
;; ANSWER SECTION:
gmail.com. 408 IN MX 10 alt1.gmail-smtp-in.l.google.com.
gmail.com. 408 IN MX 20 alt2.gmail-smtp-in.l.google.com.
gmail.com. 408 IN MX 5 gmail-smtp-in-v4v6.l.google.com.
gmail.com. 408 IN MX 40 alt4.gmail-smtp-in.l.google.com.
gmail.com. 408 IN MX 30 alt3.gmail-smtp-in.l.google.com.
lucab@galatea:~$ dig any gmail-smtp-in-v4v6.l.google.com.
;; QUESTION SECTION:
;gmail-smtp-in-v4v6.l.google.com. IN ANY
;; ANSWER SECTION:
gmail-smtp-in-v4v6.l.google.com. 259 IN AAAA 2a00:1450:8005::1a
gmail-smtp-in-v4v6.l.google.com. 205 IN A 173.194.67.27
Looking at my mailserver logs, only inbound so far.
Aquì estamos! 