https://support.google.com/a/answer/174124
1. whois
Email authentication requirements for sending to Gmail accounts Google performs checks on messages sent to Gmail accounts to verify messages are authenticated. To help ensure these messages are delivered as expected, set up email authentication for your domain. We recommend you always set up SPF and DKIM to protect your organization’s email, and to meet the authentication requirements described in Email sender guidelines. If you use an email service provider: Verify that your provider's authentication methods meet the requirements in Email sender guidelines. If you regularly forward email: Follow our Best practices for forwarding email to Gmail to help ensure messages are delivered as expected.