Mails are not always send When a user creates an account, sometimes all the mails are sent, sometimes not all, and sometimes no mails are sent but they are returned on the site-owner mail address. All the mails are considered as spam and i don't know why. On the returned mail i get this (I have no error in Drupal report and logs) :

This is the mail system at host "my host".

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

               The mail system


&lt;"site mail">: host
    gmail-smtp-in.l.google.com[2a00:1450:4013:c01::1a] said: 550-5.7.1
    [2001:41d0:d:363d::] 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
    https://support.google.com/mail/?p=IPv6AuthError for more information 550
    5.7.1 . w82si7064886wma.125 - gsmtp (in reply to end of DATA command)

Final-Recipient: rfc822; "targeted email"
Action: failed
Status: 5.7.1
Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 550-5.7.1 [2001:41d0:d:363d::] 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
    https://support.google.com/mail/?p=IPv6AuthError for more information 550
    5.7.1 . w82si7064886wma.125 - gsmtp

----- * MAIL RESUME * ----- 
...

What can i do to fix that ?
The error message you have provided indicates that you do not have a reverse PTR record in your DNS for the IP address you are using, specifically in this case for IPv6.  When you look a specific IP address up it should resolve back to a known hostname which identifies it in DNS.  Some mailservers and systems will not accept email from hosts without a valid PTR record.  I would imagine that in your case you have some IPv6 records in place, and that your server is connecting to Google's SMTP servers over IPv6 but that you do not have the correct records in place.  The error message provides a link to https://support.google.com/mail/?p=IPv6AuthError which should tell you more about this error.

You should also make sure that you have correctly setup SPF for the domain that is used as the from address in your emails, you may need to add an SPF record, or update the existing one for the domain so that your webserver is trusted to send email on behalf of your domain.