A common problem for some hosting managers nowadays is related to misconfigurations around IPv6, causing problems sending emails to Google.
Despite that the best policy should be to accurately configure your DNS zone considering IPv6 (including PTR records, DKIM etc ) in some occasions the way to work around problems related to delivering emails to Google can be the full disabilitation of IPv6 on your Postfix server. Especially if we consider that many providers still assign you dynamic IPv6 addresses.
What is the problem in first place ?
550-5.7.1 [xxxx:xxx:xxx:xxx::xx xx] 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. oiuaspoiud3iuwlkns.108 - gsmtp
How to work this around without mangling for good your IPv6 problems ?
Not using IPv6 on your Plesk with Postfix server:
Enforce Postfix to use IPv4 by specifying the inet_protocols parameter in its main configuration file:
.a using your favorite text editor edit the file
.b find and change inet_protocols from =all to =ipv4
inet_protocols = ipv4
.c restart your Postfix server
# /etc/init.d/postfix restart
Now use one email account on your server and send to some Gmail or G Suite email address. In your mail log file (usually found in /usr/local/psa/var/log/maillog) you can now see your emails being delivered to Google:
Oct 9 15:32:41 hostname postfix/smtp: 4907C67338: to=<firstname.lastname@example.org>, relay=gmail-smtp-in.l.google.com[18.104.22.168]:25, delay=0.98, delays=0.17/0.01/0.3/0.48, dsn=2.0.0, status=sent (250 2.0.0 OK 1476023561 u72si14410604lfg.289 - gsmtp)