Gmail already guide the most detail here. However, we need to repeat the most important points that we can do by ourselves to solve such issue.
When you found your mail always go to Gmail junk folder, please do following:
1. Check if you have Added SPF record for your domain. If you are using our email service, our system should have add it by default. You can check from Control Panel --->DNS.
3. Generate DMARC record online and Add DMARC record for your domain.
4. Test by send email to your personal Gmail account now.
5. If still does not work, please kindly contact with your Email service provider now.