Troubleshooting the safety tip for fraud detection checks
If you are getting a safety tip that says "The sender failed our fraud detection checks and may not be who they appear to be", then the sender failed to pass either DKIM or SPF authentication checks. The best method to resolve this is for the sender to authorize themselves. If the sender is sending on your behalf, you need to authorize them by adding the sender's IP address to your SPF record.
See Troubleshooting the red (suspicious) safety tip for fraud detection checks for more info.
Here are some other links that can help: