Very recently I have had users starting to get this error. We are in transition from postini to google apps. We don't use postini for outgoing and incoming email seems to be unaffected as does email to other domains but gmail.com.
This is the error that my users are getting when sending to gmail addresses.
mx.google.com rejected your message to the following e-mail addresses:
mx.google.com gave this error:
16] Our system has detected that this message does not meet IPv6 sending guidelines regarding PTR records and authentication. Please review https://support.google.com/mail/answer/81126
for more information. u6si2399853obi.28 - gsmtp
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery.