Found this on my Poundhost server when trying to send emails to Gmail - "The sender does not meet basic ipv6 sending guidelines of authentication and rdns resolution of sending ip. Please review https://support.google.com/mail/answer/81126for more information."
Strangely, turned out to be that IPV6 was switched on the network interface. To fix it, edit /etc/sysctl.conf and add :
The restart networking and postfix mailer:
Make sure /etc/hosts does not contain ::1 - as this in IPv6 address and will cause postfix emailer to error. Just comment it out and restart postfix.
You can set the hostname to whatever after that.
One thing to check is that Reverse Lookups are working correctly - that there is a PTR record for your IP address.
if this doesn't return a valid domain name, then you'll have to ask your hosting company to add one for that IP address (can't be done by using plain DNS).
Got this from: http://en.kioskea.net/faq/759-ubuntu-disabling-ipv6-support