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Unable to send an email to Gmail from a Plesk server: Our system has detected that this message does not meet IPv6 sending guidelines regarding PTR records

Plesk server has IPv6 address. When an email message is sent from a Plesk server to a Gmail account, it fails to be delivered.
The following error message appears in /var/log/maillog:
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 sender receives an email with 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

Gmail Guidelines for IPv6 are not met: the IPv6 address does not have a PTR record.

Solution I: Contact your hosting provider to set up a reverse DNS for IPv6 address.

Solution II: Apply one of the following instructions:

Configuring mail server to use IPv4 only (Postfix)

Log in to Plesk.
Go to Tools & Settings > Mail Server Settings
Under Outgoing mail mode select Send from the specified IP addresses:
1. for IPv4 set the desired IPv4 address from the dropdown menu
2. for IPv6 select None
Click OK.

Disabling IPv6 addresses on the server

Log in to Plesk.
Go to Tools & Settings > IP Addresses > click on the Sites number next to IPv6. If there are domains hosted on IPv6, go to Domains > example.com > Web Hosting Access and switch these domains to IPv4 only.
When you are sure no domains are using IPv6 in Plesk, connect to the Plesk via SSH and open the file /etc/sysctl.conf in a text editor. In this example, we are using the vi editor:
# vi /etc/sysctl.conf
Add the following lines:
# grep ipv6 /etc/sysctl.conf
net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6=1
Apply the changes with the following command:
# sysctl -p
Remove IPv6 addresses from Plesk: Click Reread IP at Tools & Settings > IP Addresses.

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