This is not a bug.

You need to configure your domain and add SPF and DKIM records (and DMARC). You have a link with more information in the email you received.

Quote Originally Posted by giancalr View Post
550-5.7.26 ip: [157.90.135.249].To best protect our users from spam, the
550-5.7.26 message has been blocked. Please visit
550-5.7.26 https://support.google.com/mail/answ...authentication for more
550 5.7.26 information. q25-20020a056000137900b002bc214edbd6si22510995wrz.80 - gsmtp
The link will take you to Google and you'll see a detailed explanation on how to do this. If you can't do it yourself, your host might be able to help you with it. But give it a try, it's not as complicated as it seems at first.