Sending mail error

I get only with owncloud an error when I send mail to my users.
It work well with other script like wordpress but not with owncloud.
can you help me ?

Steps to reproduce
1.with the plugin registration when an user make his registration.
2.I get no mail.
3.and I get this error message:

Feb  1 20:02:12 sampserver postfix/smtp[26214]: 11A779FC01: to=<>,[2a00:1460:400c:c07::1a]:25, delay=0.37, delays=0.11/0.02/0.04/0.21,
dsn=5.7.1, status=bounced (host[2a00:1460:400c:c07::1a] said: 550-5.7.1 
Our system has detected that this message does 550-5.7.1 not meet IPv6 sending guidelines regarding PTR records and 550-5.7.1 authentication. 
Please review 550-5.7.1 for more information 550 5.7.1 .31si25665663wre.267 - gsmtp (in reply to end of DATA command))

Expected behaviour
to send email.

Actual behaviour
nothing. just a log error message.

Server configuration
Operating system:debian jessie
Web server:apache 2
PHP version:5
ownCloud version (see ownCloud admin page):9.1.3
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption): registration plugin

Did you read the error message? Did you set a correct PTR-record for your ipv6 address? It is not really a owncloud problem...