Is it stupidity or transparency since they don't know for sure it's legit? Just be a good netizen and use SPF and DKIM for that.
https://support.google.com/mail/answer/1311182?hl=en
http://stackoverflow.com/questions/8236312/how-to-remove-via...
https://support.google.com/mail/answer/1311182?hl=en
http://stackoverflow.com/questions/8236312/how-to-remove-via...
https://www.stavros.io/posts/how-properly-configure-google-a...