Mails are not always send
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.