Hi Folks, 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: host gmail-smtp-in.l.google.com [2a00:1450:4008:c01::1a]: 550-5.7.1 [2402:6000:1000:x::x] Our system has detected that this message does 550-5.7.1 not meet IPv6 sending guidelines regarding PTR records and 550-5.7.1 authentication. Please review 550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_errorfor more 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. Cheers, Blair