>>Symptoms
Plesk server has IPv6 address. When an email message is sent from the server to a Gmail account it fails to deliver.
In /usr/local/psa/var/log/maillog on Linux or in C:\Program Files (x86)\Parallels\Plesk\Mail Servers\Mail Enable\Logging\SMTP\SMTP-Debug.log on Windows the following error appears:
ipv6_failed_after_I_sent_the_message./Remote_host_said:_550-5.7.1_[ipv6______16]_Our_system_has_detected_that_this/550-5.7.1_message_does_not_meet_IPv6_sending_guidelines_regarding_PTR_records/550-5.7.1_and_authentication._Please_review/550-5.7.1_https://support.google.com/mail/?p=ipv6_authentication_error_for_more/550_5.7.1_information._hg7si388602wjb.61_-_gsmtp/
The email message sender receives following message:
Remote host said: 550-5.7.1 [2a00:14e8:211:2123::2 16] Our system has detected that this
550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records
550-5.7.1 and authentication. Please review
550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_error for more
550 5.7.1 information. la8si618732wjb.94 – gsmtp
>>Cause
Gmail Guidelines for IPv6 are not met.
>>Resolution
Verify that the IPv6 address has a PTR-record from any Linux shell:
# dig -x 2001:db8:f61:a1ff:0:0:0:80 |grep PTR
;0.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.f.f.1.a.1.6.f.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR example.com
Verify that an answer for PTR record query matches the IP obtained via the forward DNS resolution of the hostname specified in the PTR record:
# dig example.com|grep IN|grep A
203.0.113.2
2001:db8:f61:a1ff:0:0:0:80
In case these conditions are not met, contact your ISP.
>>Workaround
On Linux, Postfix can be configured to use only IPv4 addresses:
Connect to the server via SSH.
Check the current protocols:
# postconf inet_protocols,
inet_protocols = all
In /etc/postfix/main.cf, replace inet_protocols=all with inet_protocols=ipv4 using a text editor (for example, vi):
# grep ‘inet_protocols’ /etc/postfix/main.cf
add “inet_protocols = ipv4” and add ‘#’ to “inet_protocols = all”
inet_protocols = ipv4
#inet_protocols = all
Restart Postfix:
# service postfix restart
Ensure that the configuration is updated:
# postconf inet_protocols
inet_protocols = ipv4
Source: V. CHUKWUKA