Just got caught out with this on my personal box today. It looks as if
Gmail is now checking RDNS on any inbound ipv6 smtp connections and
rejecting with 500 error if it doesn't match.
example fail -
SMTP error from remote mail server after end of data:
550-5.7.1 [2402:6000:1000:x::x] Our system has detected that this
550-5.7.1 not meet IPv6 sending guidelines regarding PTR records and
550-5.7.1 authentication. Please review
550 5.7.1 information. oq6si4802967bkb.182 - gsmtp
So if you have ipv6 enabled on your mail servers and haven't yet set some
RDNS for them.. now is the time.